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DAVIS 


THE  ABSORPTION  SPECTRA  OF  SOLUTIONS  AS  AFFECTED  BY 

TEMPERATURE  AND  BY  DILUTION:  A  QUANTITATIVE 

STUDY  OF  ABSORPTION  SPECTRA  BY  MEANS 

OF  THE  RADIOMICROMETER 


By 


HARRY  C.  JONES  and  J.  SAM  GUY 


WASHINGTON,  D.  C. 

PXJBUSHBD  BY  THE  CaKNBQIE  INSTITUTION  OP  WASHINGTON 

1913 


IJNIVERSITY  OF  CALIFORNIA? 
DAVIS 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
Publication  No.  190 


PRESS   OF   GIBSON   BROTHERS,  INC-, 
WASHINGTON,  D.  C. 


PREFACE. 


The  effect  of  high  temperatures  on  the  absorption  spectra  of  nonaqueous 
solutions  was  worked  out  in  the  Johns  Hopkins  Univejsity  and  pubhshed 
in  monograph  No.  160.  By  means  of  a  form  of  apparatus  devised  by  Dr. 
Strong,  this  work  has  now  been  extended  to  aqueous  solutions  and  the 
results  are  herein  recorded. 

Our  previous  work  on  the  absorption  spectra  of  solutions,  which  has  now 
been  in  progress  continuously  for  eight  years,  had  shown  that  the  effect  of 
dilution  on  absorption  spectra  is  much  less  than  had  hitherto  been  supposed. 
A  form  of  apparatus  and  method  of  procedure  were  worked  out  by  Professor 
Anderson,  one  of  my  former  coworkers  in  this  field,  and  this  method  has  been 
applied,  with  unusual  skill,  by  Dr.  Guy,  to  the  effect  of  dilution  on  absorp- 
tion spectra.  The  results  that  he  has  obtained  are  also  recorded  in  this 
monograph. 

The  grating  spectrograph  as  a  means  of  studying  absorption  spectra 
has  now  supplanted  the  prism  spectroscope.  The  grating  spectrograph, 
however,  has  its  limitations.  The  results  are  photographed.  This  means 
that  the  method  is  limited  to  the  range  of  the  photographic  plate.  This  is, 
for  the  best  plates,  from  about  0.2/x  to  0.8/x.  It  is,  however,  very  desirable 
to  study  absorption  spectra  in  the  region  of  wave-lengths  which  are  much 
greater  than  O.Sjjl.  For  this  purpose,  some  method  had  to  be  devised  which 
did  not  make  use  of  the  photographic  plate.  The  radiomicrometer  was  the 
obvious  instrument  to  use,  if  it  could  be  built  sufficiently  sensitive  and  at 
the  same  time  with  sufficiently  short  period.  This  has  been  accomplished 
by  Dr.  Guy. 

With  this  instrument  the  absorption  spectra  of  a  number  of  salts  have 
already  been  mapped,  and  some  surprising  results  have  been  obtained  in 
reference  to  the  relative  absorption  of  free  water  as  compared  with  water 
of  hydration. 

It  gives  me  pleasure  to  express  our  thanks  to  Dr.  E.  J.  Shaeffer,  who  has 
assisted  in  making  the  radiomicrometer  readings  during  the  second  half 
of  the  past  year,  and  who  has  also  aided  in  the  chemical  work.  Dr.  Shaeffer 
will  continue  the  work  on  the  absorption  spectra  of  solutions,  using  the 
radiomicrometer.  We  are  especially  indebted  to  Professor  A.  H.  Pfund  for 
a  large  number  of  valuable  suggestions,  and  for  frequent  advice  during  the 
progress  of  this  week.  Professor  J.  S.  Ames  has  kindly  placed  ample  space 
at  our  disposal  for  carrying  out  this  investigation. 

I  am  deeply  grateful  to  the  Carnegie  Institution  of  Washington  for 
financial  aid  in  carrying  out  this  entire  work,  and  in  publishing  the  results 
obtained.  Without  this  aid,  the  work  recorded  in  monographs  Nos.  60, 
110,  130,  160,  and  herein  could  not  have  been  done. 

Harry  C.  Jones. 

m 


CONTENTS. 


Chapter  I.     Introduction 1 

Chapter  II.    The  Absorption  Spectra  of  Aqueous  Solutions  as  Affected 

BY  Temperature 5 

The  Making  of  a  Spectrogram 7 

Neodymium  Chloride  in  Water 8 

Neodymium  Bromide  in  Water 9 

Neodymium  Nitrate  in  Water 9 

Neodymium  Acetate  in  Water 11 

Neodymium  Sulphate  in  Water 12 

Cobalt  Chloride  in  Water 12 

Praseodymium  Chloride  in  Water 13 

Praseodymium  Nitrate  in  Water 13 

Uranyl  Nitrate  in  Water 14 

Uranyl  Sulphate  in  Water 14 

Uranyl  Acetate  in  Water 15 

Chapter  III.    The  Effect  of  Dilution  on  the  Absorption  of  Light  by 

Solutions 17 

Making  a  Dilution  Spectrogram 18 

Neodymium  Chloride  in  Water 18 

Neodymium  Bromide  in  Water 20 

Neodymium  Nitrate  in  Water 20 

Neodymium  Sulphate  in  Water 21 

Neodymium  Acetate  in  Water 22 

Praseodymium  Chloride  in  Water 23 

Praseodymium  Nitrate  in  Water 24 

Uranyl  Chloride  in  Water 24 

Uranyl  Bromide  in  Water 24 

Uranyl  Nitrate  in  Water 25 

Chapter  IV.     The  Absorption  Spectra  of  Aqueous  Solutions  of  Certain 

Salts  of  Neodymium  as  Studied  by  Means  of  the  Radiomicrometer  .  29 

Method  of  Procedure 31 

Discussion  of  the  Results 36 

Possible  Explanation 38 

Chapter  V.     The  Absorption  of  Light  by  Water  Changed  in  the  Presence 
OF  Strongly  Hydrated  Salts,  as  Shown  by  the  Radiomicrometer. 

New  Evidence  for  the  Solvate  Theory  of  Solution 43 

Absorption  of  Free  and  Combined  Water 43 

Hydrated  and  Nonhydrated  Substances 44 

Method  of  Procedure 44 

Results 45 

Discussion  of  the  Results 52 

Explanation  of  the  Results 54 

Chapter  VI.    Absorption  Spectra  of  a  Number  of  Salts  as  Measured  by 

Means  of  the  Radiomicrometer 61 

Mode  of  Procedure 62 

Description  of  Cells  Used 63 

Discussion  of  Tables  and  Curves 65 

Neodymium  Chloride  in  Water 65 

Neodymium  Nitrate 72 

Neodymium  Acetate 74 

Praseodymium  Chloride 76 

Praseodymium  Nitrate 78 

Nickel  Chloride 79 

Nickel  Nitrate 80 

Nickel  Sulphate 80 

Salts  of  Cobalt 81 

Chapter  VII.    General  Summary  of  Results 85 

V 


THE  ABSORPTION  SPECTRA  OF  SOLUTIONS  AS  AFFECTED  BY 

TEMPERATURE  AND  BY  DILUTION:  A  QUANTITATIVE 

STUDY  OF  ABSORPTION  SPECTRA  BY  MEANS 

OF  THE  RADIOMICROMETER 


By 
HARRY  C.  JONES  and  J.  SAM  GUY 


vn 


CHAPTER  I. 

INTRODUCTION. 

An  investigation  of  the  effect  of  temperature  on  the  absorption  spectra 
of  certain  solutions  has  already  been  carried  out  by  Jones  and  Strong.^ 
The  apparatus  used  was  devised  by  Professor  John  A.  Anderson,^  who 
worked  somewhat  earlier  with  Jones  on  the  absorption  spectra  of  solutions. 
The  solutions  were  heated  in  an  open  vessel,  and  the  temperature  could, 
of  course,  not  be  raised  much  above  100°  F.  It  was  found  that,  even  over 
this  range  of  temperature,  the  effect  of  rising  temperature  was  to  cause  the 
general  absorption  of  any  salt  in  water  to  increase,  and  also  to  cause  the 
bands  to  broaden  and  become  more  diffuse.  The  results  were  entirely 
unambiguous  so  far  as  they  went,  but  were  limited  by  the  boiling-points  of 
the  solutions  in  question.  Indeed,  it  was  not  possible  to  work  quite  up  to 
the  boiling-point  of  the  solution,  since  the  change  in  the  concentration  of 
the  solution  resulting  from  boiUng  would  have  been  too  great,  and  there 
would  have  been  too  much  gas  formed  on  the  quartz  windows  through  which 
the  light  was  to  pass. 

We  wanted  to  study  the  effect  of  rise  in  temperature  on  the  absorption 
spectra  of  solutions  to  as  high  temperatures  as  it  was  possible  to  go.  For 
this  purpose  closed  forms  of  apparatus  devised  by  Anderson'  and  by  Strong* 
were  employed  by  Jones  and  Strong^  for  nonaqueous  solutions.  The  appa- 
ratus consisted  of  a  gold-plated  steel  tube,  whose  ends  were  closed  with 
glass  windows.  This  worked  very  well  with  nonaqueous  solvents  up  to 
temperatures  of  approximately  200°  C.  Usually  before  this  temperature 
was  reached  a  precipitate  formed  in  the  tube,  which  prevented  work  at 
higher  temperatures. 

Some  interesting  results  were  obtained  at  the  higher  temperatures  with 
this  apparatus.  The  general  effect  of  rise  in  temperature  is  to  deepen  the 
color  of  the  solution  of  an  inorganic  salt.  This  is  usually  due  to  a  widening 
of  the  absorption  bands.  For  details  in  reference  to  the  effect  of  tempera- 
ture on  the  absorption  of  light  by  nonaqueous  solutions,  reference  must  be 
had  to  the  Carnegie  Institution  of  Washington  monograph,^  where  the 
results  in  question  are  published  in  full. 

The  apparatus  used  by  Jones  and  Strong  for  nonaqueous  solutions  did 
not  work  satisfactorily  for  solutions  in  water  as  the  solvent.  The  water 
vapor,  under  the  high  pressure  produced  within  the  apparatus,  worked  its 

iCam.  Inst.  Wash.  Pub.  130.     Amer.  Chem.  Joum.,  43,  37,  97  (1910);  45,  1,  113  (1911). 

2Cara.  Inst.  Wash.  Pub.  110,  p.  20.     Amer.  Chem.  Journ.,  41,  276  (1909). 

3  Cam.  Inst.  Wash.  Pub.  160,  p.  28.     Amer.  Chem.  Joum.,  47,  30  (1912). 

^Carn.  Inst.  Wash.  Pub.  160,  p.  29.     Amer.  Chem.  Journ.,  47,  30  (1912). 

5  Cam.  Inst.  Wash.  Pub.  160.     Amer.  Chem.  Journ.,  47,  27,  126  (1912). 

•Cam.  Inst.  Wash.  Pub.  160. 


2  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

way  through  the  layer  of  gold  laid  down  on  a  layer  of  copper  electrolytically, 
rusted  the  steel,  and  caused  the  separation  of  the  gold  from  the  steel  walls. 
To  avoid  this,  the  apparatus,  which  was  designed  by  Dr.  Strong,^  was  made 
of  brass  and  will  be  described  in  some  detail  in  this  monograph.  It  was 
plated  electrolytically  with  gold  and  this  adhered  firmly  to  the  brass,  even 
when  the  aqueous  solution  contained  in  the  apparatus  was  heated  to  200°  C. 
We  could  work  as  satisfactorily  with  this  apparatus  with  aqueous  solutions 
as  with  the  former  apparatus  with  nonaqueous  solutions. 

The  work  described  in  this  monograph  on  absorption  spectra  of  aqueous 
solutions  at  high  temperatures  was  all  carried  out  in  the  gold-plated  brass 
apparatus.  The  results  obtained  and  the  bearing  of  these  results  on  the 
nature  of  solution  will  be  discussed  later  in  this  monograph.  Suffice  it  to 
say  here  that  up  to  200°  the  effect  of  temperature  on  the  absorption  spectra 
of  aqueous  and  nonaqueous  solutions  has  now  been  studied  pretty  exten- 
sively on  a  large  number  of  salts  and  a  fairly  large  number  of  solvents. 

The  effect  of  dilution  on  the  absorption  spectra  of  solutions  was  taken 
up  with  the  following  idea  in  mind:  It  was  long  a  question  as  to  what  is 
the  nature  of  the  absorber  of  light,  say  in  aqueous  solutions.  It  was  at  one 
time  supposed  that  chemical  molecules  were  the  absorbers,  since  these  were 
regarded  as  the  ultimate  units  in  solution.  It  was  supposed  that  the  mole- 
cules were  thrown  into  resonance  by  certain  wave-lengths  of  light,  and  that 
these  were,  consequently,  stopped;  while  the  remaining  wave-lengths  passed 
through  the  solution  and  gave  to  it  its  characteristic  color. 

When  the  theory  of  electrolytic  dissociation  was  proposed  in  1886,  the 
view  as  to  the  nature  of  solution  of  electrolytes  underwent  a  serious  change. 
When  electrolytes  were  dissolved  in  water,  or  in  any  other  dissociating 
solvent,  they  dissociated  into  charged  parts  or  ions,  and  these  were  the 
ultimate  units  in  solution.  If  the  solution  was  fairly  concentrated  we  had 
both  ions  and  undissociated  molecules  in  the  solution,  and  the  question  in 
such  cases  was,  which  is  the  absorber? 

It  was  further  recognized  that  a  dilute  solution  of  salt  often  has  very 
different  color  from  a  concentrated  solution;  and,  moreover,  solutions  of 
nonelectrolytes  are  often  colored,  i.  e.,  have  the  power  to  absorb  certain 
wave-lengths  of  light  and  to  allow  others  to  pass  on  through.  It  was  sup- 
posed, then,  that  molecules  have  the  power  to  absorb  light,  and  ions  also 
have  absorbing  power.  When  a  concentrated  and  a  dilute  solution  of  an 
electrolyte  had  the  same  absorption  spectrum — the  same  color — it  was 
supposed  that  the  chemical  molecule  and  the  ions  resulting  from  it  had  the 
same  absorption.  When  the  dilute  solution  had  a  different  color  from  the 
concentrated  solution,  it  was  thought  that  the  ions  were  the  chief  absorbers 
of  light.  And  since  it  frequently  happens  that  a  dilute  solution  of  an  elec- 
trolyte has  a  very  different  color  from  a  more  concentrated  solution,  it 
was  supposed  that  in  dilute  solutions  of  electrolytes  the  ions  are  the  chief 
absorbers  of  light;,  since  in  very  dilute  solutions  of  electrolytes  there  are 

1  Cam.  Inst.  Wash.  Pub.  160.     Amer.  Chem.  Journ.,  47,  30  (1912). 


Introduction.  3 

mainly  ions  and  practically  no  molecules  present,  it  is  obvious  that  in  such 
solutions  it  is  not  the  molecules  which  are  absorbing  light.  It  must  be  the 
ions,  since  these  are  the  only  units  present,  or  something  contained  within 
the  ions.  This  was  the  view  of  absorption  of  light  introduced  by  the  theory 
of  electrolytic  dissociation. 

We  have  now  gone  much  farther  than  this.  We  now  know  that  the  ions 
are  not  the  ultimate  units  in  a  solution  of  an  electrolyte.  The  simplest  ion 
is  very  complex.  It  is  made  up  of  a  large  number  of  electrons,  which  are 
unit  negative  charges  of  electricity.  There  is  every  reason  to-day  to  believe 
that  the  electrons  are  the  real  absorbers  of  light,  are  the  units  which  are 
thrown  into  resonance  by  the  various  wave-lengths  of  light.  Granting  this, 
there  is  still  a  difference  between  an  ion  and  the  atom  or  atoms  from  which 
it  was  formed.  An  ion  contains  one  or  more  free  electrons  within  it  or  on 
it,  i.  e.y  one  or  more  negative  charges  than  would  correspond  to  the  positive 
electricity  within  the  atom.  It  would  be  interesting  to  know  whether  the 
free  electron  or  electrons  upon  the  ion  have  anything  to  do  with  its  power  to 
absorb  light.  This  can  be  tested  by  studying  the  absorbing  power  of  mole- 
cules and  then  the  absoi-ption  of  light  by  the  ions  which  are  formed  when 
these  molecules  dissociate.  It  was  with  this  idea  in  mind  that  the  second 
chapter  of  the  work  described  in  this  monograph  was  undertaken. 

A  concentrated  solution  of  a  salt  contains  many  molecules,  and  if  the  solu- 
tion is  sufficiently  concentrated  there  are  chiefly  molecules  and  only  a  few 
ions  present.  As  the  dilution  is  increased  the  dissociation  increases;  the 
number  of  molecules  becomes  less  and  less  and  the  number  of  ions  greater 
and  greater.  The  problem,  then,  is  to  photograph  the  absorption  spectrum 
of  a  very  concentrated  solution  of  a  salt,  the  layer  being,  say,  0.5  cm.  deep. 
Then  take  the  spectrum  of  a  more  dilute  solution  of  the  same  salt;  if  the 
dilution  is  increased  100  times  the  depth  of  layer  used  would  be  50  cm. 
Under  these  conditions  there  would  be  the  same  number  of  parts  of  dis- 
solved substance  in  the  path  of  the  beam  of  light;  in  the  second  case  there 
would  be  more  ions  and  less  molecules  than  in  the  first.  By  comparing  the 
two  spectra  we  could  see  whether  there  is  any  difference  between  the 
absorbing  power  of  ions  and  molecules,  ^.  e.,  whether  the  free  electrons  upon 
the  ions  have  anything  to  do  with  their  power  to  absorb  light.  We  then 
took  another  step,  increasing  the  dilution  of  the  second  solution  five  times 
and  also  increasing  the  depth  of  the  layer  of  the  solution  through  which  the 
light  passed  five  times,  i.  e.,  making  the  depth  250  cm.  This  second  diluting 
still  further  reduced  the  number  of  molecules  present  and  increased  the 
number  of  ions.  By  comparing  the  three  spectrograms  we  ought  to  be  able 
to  say  whether  molecules  and  ions  have  the  same  or  different  resonance 
with  respect  to  light- waves;  and,  if  it  is  different,  to  point  out  in  what  the 
difference  consists.  This  would  then  enable  us  to  determine  whether  the 
free  electrons  upon  the  ions  played  any  part  in  the  absorption  of  light. 

We  shall  see  that  ions  have  somewhat  different  absorbing  powers  from 
molecules,  and  in  what  this  difference  consists  will  appear  later  from  the 
text  and  from  the  plates. 


4  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

The  work  done  on  the  absorption  spectra  of  solutions  by  Jones  and 
Uhler,^  Jones  and  Anderson,*  and  Jones  and  Strong,^  which  extended  over 
five  years,  and  in  which  some  6,000  solutions  were  studied,  all  involved  the 
photographing  of  the  various  spectra.  In  this  way  the  positions  of  the 
various  absorption  lines  and  bands  were  determined. 

A  question  even  more  fundamental  than  the  positions  of  the  lines  and 
bands  is  their  intensities,  and  the  relative  intensities  of  different  parts  of 
the  same  band.  The  photographic  method  gave  only  a  means  of  dealing 
qualitatively  with  this  problem.  Some  general  idea  could  be  gained  of  the 
relative  intensities  of  the  various  lines  and  bands  on  the  photographic  plate, 
but  these  changed  with  the  time  of  exposure,  the  intensity  of  the  light  used, 
and  with  other  conditions,  so  that  we  were  able  to  learn  very  little  about 
the  intensities  of  the  various  lines  and  bands  by  means  of  the  photographic 
method. 

Further,  the  photographic  plate  is  sensitive  only  between  the  wave-length 
2,000  Angstrom  units  and  7,600  A.u.,^  which  is  a  comparatively  small  part  of 
the  spectrum.  It  is  especially  important  to  work  also  into  the  region  of  the 
infra-red. 

A  method  was  used  which  dealt  quantitatively  with  the  intensities  of  the 
various  lines  and  bands.  This  same  method,  instead  of  being  limited  by 
the  above-named  wave-lengths,  could  be  used  down  into  the  infra-red  to 
wave-lengths  as  great  as  20,000  a.u.  to  30,000  a.u.  Indeed,  the  method  can 
be  used  for  even  greater  wave-lengths,  if  solvents  can  be  found  that  are 
transparent  to  the  longer  waves.  This  method  involves  the  use  of  the  radio- 
micrometer. 

The  description  of  the  instrument  which  we  built,  the  method  of  work, 
and  the  results  thus  far  obtained,  will  be  found  on  pp.  29  to  93. 

iCarn.  Inst.  Wash.  Pub.  60.     Amer.  Chem.  Journ.,  37,  126,  207  (1907). 

2  Cam.  Inst.  Wash.  Pub.  110.     Amer.  Chem.  Joiu-n.,  loc.  cit. 

*Carn.  Inst.  Wash.  Pub.  130  and  160.     Amer.  Chem.  Journ.,  loc.  cit.     „ 

^Throughout  this  paper  we  have  employed  this  expression  to  designate  Angstrom  units. 


CHAPTER  II. 

ABSORPTION  SPECTRA  OF  AQUEOUS  SOLUTIONS  AS  AFFECTED 
BY  TEMPERATURE. 

Jones  and  Strong^  studied  the  effect  of  temperature  on  the  absorption 
spectra  of  various  nonaqueous  solutions  up  to  nearly  200°.  The  solutions 
were  usually  heated  until  a  precipitate  formed,  which  cut  off  the  light  and 
prevented  work  at  still  higher  temperatures.  Some  work  was  also  done  by 
Jones  and  Strong  on  the  effect  of  temperature  on  the  absorption  spectra  of 
aqueous  solutions.  This  was,  however,  not  large  in  amount  and  did  not 
extend  to  very  high  temperatures. 

The  reason  that  the  work  with  aqueous  solutions  was  not  pushed  to  higher 
temperatures  was  that  the  form  of  apparatus  then  in  use  did  not  admit 
of  it.  This  consisted  of  a  steel  tube,-  lined  on  the  inside  with  copper  and 
plated  with  gold  on  all  of  the  inner  surfaces.  This  worked  very  satisfac- 
torily with  nonaqueous  solutions,  the  gold  plate  adhering  firmly  to  the 
copper,  which,  in  turn,  remained  adherent  to  the  steel.  When  an  aqueous 
solution  was  heated  in  the  apparatus  from  100°  to  200°  the  result  was 
unsatisfactory.  The  water,  under  the  high  pressure,  forced  its  way  through 
the  copper  and  the  gold  and  rusted  the  iron,  as  has  already  been  stated. 
The  result  was  that  the  copper,  with  the  gold,  separated  from  the  steel,  and 
the  solutions,  after  heating  for  a  time,  gave  the  iron  reaction.  This  appa- 
ratus had  the  further  disadvantage,  that  when  a  precipitate  formed  with 
rising  temperature  it  was  necessary  to  open  the  entire  apparatus  and  remove 
the  glass  ends  in  order  to  clean  them. 

To  overcome  these  difficulties  the  apparatus  shown  in  fig.  A  was  con- 
structed by  Jones  and  Strong  and  used  to  study  the  effect  of  rising  tempera- 
ture on  the  absorption  spectra  of  aqueous  solutions.  The  quartz  ends  are 
fastened  into  the  ends  E'.  The  plunger  P  has  guide  grooves  instead  of 
guide  pins.  A  part  of  the  plunger  is  provided  with  screw-threads  for  remov- 
ing it.  The  entire  cap  is  removed  from  tube  T  by  unscrewing  E',  during 
which  the  quartz  end  is  untouched.  When  the  ends  are  removed  the  quartz 
window  can  be  easily  cleaned.  Gold  washers  were  inserted  between  T  and 
E'  and  between  E'  and  U. 

The  general  arrangement  of  the  apparatus  is  also  shown  in  fig.  A.  The 
cell  is  kept  in  a  horizontal  position,  so  that  any  bubbles  that  may  form  will 
rise  in  the  side  tube.  The  spectroscope,  containing  the  grating  G,  photo- 
graphic-plate holder  C,  and  slit  S,  being  kept  vertically,  a  45°  quartz  prism 
was  used  to  change  the  horizontal  beam  of  light  into  a  vertical  beam,  the 
beam  being  totally  reflected  by  the  hypothenuse  surface  of  0.  The  source 
of  light  NG  (Nemst  glower)  or  SG  (spark  gap)  was  focused  by  the  concave 

^Amer.  Chem.  Journ.,  47,  27,  126  (1912).  VWd.,  47,  30  (1912). 

b 


6 


ABSORPTION   SPECTRA   OF   SOLUTIONS 


speculum  mirror  M  on  the  slit  S.  A  similar  arrangement  was  used  for  the 
fused  silica  cell.  DTS  is  a  double-throw  switch,  by  means  of  which  either 
the  Nernst  glower  or  the  spark  gap  may  be  thrown  in  circuit.  B  is  a  bal- 
last, i^  is  a  variable  resistance,  by  means  of  which  the  current  in  the  Nernst 
glower,  as  shown  by  the  ammeter  A,  may  be  kept  constant.  OC  is  an  oil- 
condenser.  IC  is  an  X-ray  induction  coil  and  R2  is  a  resistance  in  the 
primary  circuit  of  this  coil. 


Fig.  a. 

In  a  recent  paper  Merton^  states  that  he  has  studied  the  effect  of  pressure 
on  the  absorption  spectra  of  solutions.  This  was  studied  here  by  Jones  and 
Strong  and  the  results  pubhshed  in  the  American  Chemical  Journal.-  The 
quotation  of  a  paragraph  from  our  earlier  paper  will  show  what  was  found: 

Some  preliminary  tests  were  made  with  the  cells  at  high  pressures.  The  Cailletet 
pump  belonging  to  the  Johns  Hopkins  University  was  used  for  this  purpose,  the  cell 
being  made  so  as  to  fit  into  this  piunp.  It  was  not  at  all  difficult  to  obtain  pressures  of 
200  atmospheres  with  water  and  alcohol  solutions.  Spectrograms  were  made  of  the 
absorption  spectra  of  neodymium  solutions  under  pressures  as  high  as  275  atmospheres. 
No  effect  of  pressure  was  detected.  The  work  at  high  pressures  is  easier  than  at  high 
temperatures,  on  account  of  the  fact  that  there  is  an  expansion  of  the  cell  due  to  heating. 

It  should  be  stated  that  in  all  of  the  work  on  absorption  spectra  which  has 
been  carried  out  in  this  laboratory  for  the  past  seven  years,  a  grating  spec- 
troscope has  been  used.  The  arrangement  of  the  heated  cell,  the  grating, 
photographic  plate,  etc.,  will  now  be  discussed. 

1  Proc.  Roy.  Soc.,  (A)  87,  146.        « Amer.  Chem.  Joum.,  47,  32,  Janiiary  1912, 


AS  AFFECTED   BY  TEMPERATURE.  7 

THE  MAKING  OF  A  SPECTROGRAM. 

The  apparatus  used  throughout  the  entire  study  of  the  effect  of  high 
temperature  has  already  been  discussed.  Two  cells  were  used,  one  10  cm. 
and  the  other  1  cm.  in  length,  both  having  the  same  general  design  and 
differing  only  in  length. 

The  cell,  placed  in  a  bath  suitable  for  keeping  the  temperature  constant, 
was  arranged  as  indicated  in  the  diagram,  and  the  source  of  light  so  located 
that  the  rays,  reflected  from  a  concave  mirror,  passed  longitudinally  through 
the  cell  and  formed  an  image  of  the  Nernst  glower  on  the  slit  of  the  camera. 
The  position  of  the  prism  is  so  adjusted  as  to  fill  the  grating  uniformly  with 
light.  Holding  the  eye  directly  above  the  grating,  in  a  position  later  to  be 
occupied  by  the  photographic  plate,  we  could  easily  tell  when  the  light  was 
falling  properly  upon  the  grating.  When  the  cell  was  correctly  adjusted 
the  lights  were  extinguished  and  the  photographic  plate  inserted.  With 
the  plate  in  position,  the  light  was  turned  on  and  an  exposure  made  at  room 
temperature.  The  position  of  the  plate  was  then  moved  a  given  distance, 
and  the  temperature  of  the  cell  raised  very  slowly,  this  process  being  repeated 
at  intervals  of  about  20°  or  25°. 

It  is  clear  that,  with  such  pressures  as  are  developed  by  heating  water  to 
200°,  it  is  very  difficult  to  obtain  a  tight  joint  between  glass  and  metal. 
This  difficulty,  however,  has  been  partly  overcome  by  the  special  form  of 
apparatus  designed  by  Dr.  Strong  and  described  on  page  6  of  this  mono- 
graph. We  were  not  able  to  secure  a  closing  that  would  hold  above  200°, 
but  once  a  good  closing  was  secured  it  was  not  necessary  to  remove  the  ends 
for  several  operations. 

Great  care  had  to  be  taken  in  heating  the  cell,  on  account  of  the  difference 
in  expansion  of  the  glass  ends,  and  the  metal  in  contact  with  them.  When 
the  temperature  was  raised  more  than  40°  an  hour  the  glass  ends  usually 
broke.  At  such  high  temperatures  as  we  were  employing  the  glass  was 
rapidly  attacked  by  the  water;  later,  when  we  were  using  the  clear  uviole 
glass,  a  single  heating  rendered  the  glass  ends  almost  opaque,  especially  if 
they  were  allowed  to  stand  for  any  length  of  time. 

It  was  found  that  in  many  cases  precipitates  would  appear  in  the  cell 
at  a  temperature  slightly  above  100°.  This  precipitate,  however,  formed 
rapidly,  once  it  began,  and  almost  as  quickly  disappeared.  By  properly 
regulating  the  intervals  at  which  exposures  were  made,  the  effect  of  the 
precipitate  could  be  avoided;  hence  this  effect  does  not  appear  on  any  of 
the  strips  photographed. 

It  is  probable  that  slight  hydrolysis  took  place  at  first,  as 

2NdCl3  +  3H2O  =  2Nd(OH)3  +  3HC1 

The  presence,  then,  of  a  slight  excess  of  hydrochloric  acid  would  hinder  the 
reaction  in  the  direction  indicated  above  by  the  arrow.  Since  most  hydrox- 
ides lose  water  at  temperatures  above  100°,  it  is  possible  that  the  following 
reaction  would  take  place: 

2Nd(0H),=Nd«0a  +  3H,0 


8  ABSORPTION   SPECTRA   OF   SOLUTIONS 

The  neodymium  oxide,  being  heavy  and  very  slightly  soluble  in  water, 
settles  to  the  bottom  of  the  cell,  and  the  solution  clears  up.  In  this  way 
it  is  evident  that  the  solution  becomes  slightly  more  dilute  as  the  tempera- 
ture is  raised;  but  this  would  lessen  the  number  of  absorbers  in  the  path  of 
the  beam  of  light,  and  thereby  produce  a  narrowing  of  the  bands  and  could 
only  decrease  the  effect  indicated  on  the  plates.  This  antagonistic  influence 
could  certainly  not  cause  a  widening  of  the  absorption  bands,  with  rise  in 
temperature. 

NEODYMIUM  CHLORIDE  IN  WATER.     (See  Plate  1.) 

The  solution  whose  spectrum  is  given  in  section  A  was  saturated,  the 
depth  of  absorbing  layer  being  1  cm.  The  temperatures,  beginning  with 
the  strip  nearest  the  numbered  scale,  were  20°,  45°,  70°,  95°,  115°,  140°,  and 
165°,  respectively.  Absorption  bands  which  are  unchanged  by  the  range  of 
temperature  from  20°  to  200°  appear  at  X3800,  X4025,  X4200,  X4325,  X4440, 
X4600,  X4690,  X4750  and  X4820.  The  double  band  from  X5050  to  X5270  is 
only  slightly  affected,  if  at  all. 

The  two  most  interesting  absorption  bands  are  those  whose  centers  are 
near  X4275  and  X5800.  The  former  of  these  in  strip  1  is  very  sharp  and 
intense,  though  only  a  few  a.u.  wide.  Both  edges  were  well  defined.  As 
the  temperature  is  raised  the  violet  edge  remains  very  sharp,  while  a  rapid 
shading  off  of  the  red  edge  takes  place.  At  a  glance  the  band  appears  to  be 
less  intense  in  the  higher  temperature  strips,  but  on  close  examination  it  is 
seen  to  be  more  diffuse,  the  red  edge  diffusing  over  a  range  of  about  20  a.u. 
at  the  highest  temperature.  This  is  exactly  in  accord  with  what  Jones  and 
Anderson^  had  found.  They  showed  that  when  the  number  of  molecules 
in  the  path  of  light  was  kept  constant,  this  band  remained  practically  con- 
stant; while  it  has  been  shown  by  Jones  and  Anderson  and  by  ourselves  that 
this  band  changes  with  dilution,  being  more  intense  in  the  most  concen- 
trated solution. 

The  X5800  band  is  affected  most  by  temperature  as  well  as  by  dilution. 
In  strip  1  this  band  is  about  200  a.u.  wide,  the  width  increasing  regularly 
as  the  temperature  is  raised,  until  at  the  highest  temperature  it  is  over 
250  A.U.,  or  there  is  a  total  widening  of  50  a.u.  The  violet  edge  remains 
perfectly  sharp,  while  the  shading  is  toward  the  red  end  of  the  spectrum. 

It  occurred  to  us  that  whatever  effect  might  be  produeed  by  a  rise  in 
temperature,  if  it  was  a  true  temperature  effect,  the  reverse  should  happen 
when  the  solution  was  allowed  to  cool. 

With  this  in  view  B  was  made.  The  concentration  of  the  solution  and 
the  depth  of  layer  photographed  in  section  B  were  exactly  the  same  as  in  A. 
In  fact,  the  same  solution  was  used.  As  soon  as  the  film  A  had  been  exposed 
with  rising  temperature,  it  was  removed  from  the  camera  and  developed. 
Without  even  allowing  the  cell  to  cool,  another  film  was  placed  in  the  camera 

«Caxn.  Inst.  Wash.  Pub.  110. 


AS   AFFECTED    BY   TEMPERATURE.  9 

and  section  B  made  with  falling  temperature.  In  B  the  temperatures  were 
165°,  140°,  115°,  95°,  and  70°,  the  highest  temperature  being  nearest  the 
numbered  scale,  which  is  not  accurately  adjusted. 

A  study  of  the  original  film  shows  changes  only  in  bands  X4275  and  X5800; 
and  this  change  is  exactly  the  reverse  of  that  shown  by  these  same  bands  in  ^. 
The  X4275  band  appears  in  strip  1,  with  a  sharp  violet  edge  and  shading  off 
toward  the  red  over  a  range  of  15  or  20  a.u.  As  we  pass  to  the  succeeding 
strips  in  the  direction  of  falling  temperature  the  red  edge  becomes  sharper 
and  sharper,  until  in  strip  5,  which  represents  the  lowest  temperature,  the 
band  assumes  its  normal  sharp  edge  on  the  red  side  and  covers  less  than  10 
A.u.  The  X5800  band  narrows  uniformly  from  the  red  end  as  the  tem- 
perature falls,  the  total  narrowing  being  about  40  a.u. 

NEODYMIUM  BROMIDE  IN  WATER.     (See  Plate  2.) 

The  concentration  of  the  solution  used  in  making  the  negative  for  A  was 
1.66  normal;  the  depth  of  cell,  1  cm.  The  temperatures,  beginning  with  the 
strip  nearest  the  numbered  scale,  were  20°,  45°,  70°,  95°,  120°,  140°,  175°,  and 
190°,  respectively.  This  plate  seems  to  have  had  just  the  proper  length  of 
exposure  for  the  given  concentration,  and  every  known  neodymium  absorp- 
tion band  appears  on  the  negative  in  excellent  condition.  With  the  bromide, 
as  with  the  chloride  discussed  in  plate  1,  only  X4275  and  X5800  show  appre- 
ciable changes  with  rise  in  temperature.  The  X4275  band,  which  has  both 
violet  and  red  edges  sharp  in  strip  1,  feathers  out  toward  the  red  end  of  the 
spectrum  as  the  temperature  is  raised. 

The  X5800  band  widens  toward  the  red  as  much  as  60  a.u.  The  concen- 
tration of  solution  used  in  making  B  was  0.166  normal,  one-tenth  that  of  A; 
the  depth  of  absorbing  layer  was  10  cm.  The  temperatures,  beginning  with 
the  strip  nearest  the  numbered  scale,  were  20°,  45°,  70°,  95°,  115°,  135°, 
155°  and  190°,  respectively.  This  is  probably  the  best  negative  produced  in 
this  part  of  the  work,  and  the  bands  X4275  and  X5800  show  well  the  char- 
acteristic changes  spoken  of  above.  The  widening  of  band  X5800,  though 
well  marked,  is  not  so  great  as  in  A,  the  total  change  being  about  40  a.u., 
as  compared  with  60  a.u.  in  the  former.  If  such  a  band  be  due  to  mole- 
cules this  is  what  we  should  expect,  since,  B  being  a  more  dilute  solution,  the 
total  number  of  molecules  is  less  than  in  A.  Hence,  any  change  associated 
with  molecules  would  be  more  clearly  apparent  in  A.  This  is  in  accord  with 
changes  produced  in  this  same  band  by  dilution. 

NEODYMIUM  NITRATE  IN  WATER.     (See  Plates  3  and  4.) 

The  solution  used  in  spectrogram  A,  plate  3,  was  saturated,  the  depth  of 
cell  being  1  cm.  The  temperatures,  beginning  with  the  strip  nearest  to  the 
numbered  scale,  were  15°,  40°,  65°,  115°,  140°,  and  165°,  respectively. 

The  exposures  were  not  as  long  here  as  in  the  previous  plates,  in  order  to 
bring  out  more  clearly  the  group  of  bands  between  X4200  and  X4800.  The 
change  in  the  X4275  band  is  here  especially  marked.  At  15°  this  band  is 
very  sharp  and  intense,  while  at  165°  it  has  become  broad  and  hazy,  being 


10  ABSORPTION  SPECTRA  OF  SOLUTIONS 

about  30  A.u.  wide.     The  X4425  band  shows  a  widening  of  about  15  a.u. 
over  the  range  shown  in  this  plate. 

The  broad  bands  with  the  centers  near  X5125  and  X5800  show  most  marked 
changes.  In  each  case  the  most  marked  change  is  almost  entirely  toward 
the  red  end  of  the  spectrum,  the  violet  edge  of  the  band  remaining  almost 
unchanged.     This  is  the  case  especially  with  the  Xo800  band. 

The  concentration  of  the  solution  used  in  B,  plate  3,  was  one-tenth  satu- 
rated, the  depth  of  the  cell  being  10  cm.  The  temperatures,  beginning 
next  to  the  numbered  scale,  were  20°,  45°,  70°,  95°,  120°,  and  145°. 

Although  the  total  number  of  absorbers  in  B  are  the  same  as  in  il,  yet  it  is 
seen  that  the  change  in  the  bands  is  far  greater  in  A,  i.  e.,  where  the  concen- 
tration is  greatest.  Only  the  X5800  band  shows  appreciable  change  in  B, 
and  even  this  does  not  widen  more  than  40  a.u. 

The  concentration  of  the  solution  used  in  making  the  negatives  of  A, 
plate  4,  was  one-tenth  of  saturation,  the  depth  of  absorbing  layer  10  cm. 
The  temperatures,  beginning  with  the  strip  nearest  the  numbered  scale, 
were  20°,  45°,  70°,  95°,  115°,  140°,  165°,  and  190°. 

Aside  from  the  shght  tendency  of  all  the  absorption  bands  to  become  a 
little  more  diffuse  at  the  higher  temperatures,  though  not  more  intense, 
there  is  no  marked  change  in  any  band  except  X4275  and  X5800.  The  former 
of  these,  as  we  go  toward  the  higher  temperatures,  remains  perfectly  sharp 
and  constant  on  its  violet  edge,  while  there  is  a  regular  shading  toward  the 
red  end  of  the  spectrum.  Again,  the  greatest  change  takes  place  in  band 
X5800,  the  violet  end  remaining  fixed  and  the  red  edge  widening  between  the 
first  and  last  strips  to  the  extent  of  about  50  a.u.  All  the  exposures  of  this 
plate  were  made  as  the  temperature  of  the  cell  was  raised. 

The  identical  solution  used  in  A  was  photographed  in  B,  plate  4,  the  cell, 
intensity  of  light-source,  and  all  of  the  apparatus  remaining  unchanged, 
the  only  difference  being  that  the  exposures  of  B  were  made  at  regular 
intervals  as  the  temperature  of  the  cell  was  lowered.  The  temperatures  of 
the  successive  strips  in  B  were,  beginning  with  the  strip  nearest  the  num- 
bered scale,  190°,  165°,  140°,  115°,  95°,  70°,  45°,  20°. 

The  original  films  shew  A  and  B  to  be  exactly  the  reverse  of  each  other. 
Just  those  changes  produced  in  A  by  a  rise  in  temperature  are  reversed  by 
the  corresponding  fall  of  temperature  in  B.  Of  course  this  is  only  qualita- 
tive, since  we  can  establish  no  definite  quantitative  relations  from  the  photo- 
graphic plates.  In  order  to  do  this,  energy  measurements  must  be  made, 
not  only  on  each  band,  but  on  different  parts  of  the  same  band.  Such  work 
is  now  in  progress.  This  would  be  very  difficult  to  do  with  a  narrow 
band  like  X4275,  but  should  be  comparatively  simple  with  band  X5800. 

Band  X4275,  which  in  strip  1  appears  broad  and  hazy  on  its  red  edge, 
gradually  acquires  the  characteristic  sharp  intense  edges  as  the  temperature 
falls,  until  in  strip  8  it  is  only  about  8  a.u.  wide.  The  total  change  in  band 
X5800  is  a  narrowing  of  about  60  a.u.  There  is  no  sudden  or  decided  change 
between  any  two  successive  strips,  but,  on  the  contrary,  so  far  as  the  photo- 
graphic plate  is  able  to  show,  the  change  is  a  gradual  one. 


AS  AFFECTED   BY   TEMPERATURE.  11 

NEODYMIUM  ACETATE  IN  WATER.     (See  Plates  5  and  6.) 

In  plate  5  we  have  photographed  the  change  in  the  absorption  bands  of 
neodymium  acetate,  produced  by  rise  in  temperature,  section  A,  and  by  the 
corresponding  lowering  of  temperature,  section  B.  The  concentration  of 
the  solution  used  for  both  negatives  was  one-tenth  of  saturation;  the  depth 
of  absorbing  layer  was  10  cm. 

The  temperatures  of  the  strips  in  A,  beginning  with  the  strip  nearest  the 
numbered  scale,  were  20°,  45°,  70°,  95°,  120°,  140°,  160°,  190°.  This  nega- 
tive shows  changes  in  bands  X4275  and  X5800;  the  former,  as  in  the  other 
plates  on  the  study  of  the  effect  of  temperature,  shows  a  marked  shading 
towards  the  red,  while  the  remainder  of  this  band  virtually  remains  fixed. 
The  X5800  band  widens  rapidly  toward  the  red,  as  the  temperature  is  raised, 
the  total  amount  being  about  80  a.u.  All  the  absorption  bands  with  the 
acetate  are  more  intense  and  broader  than  for  the  same  concentration  of  any 
of  the  other  salts  of  neodymium  studied.  The  acetate  is  not  nearly  so 
soluble  as  the  other  salts,  nor  is  the  dissociation  so  great,  yet  we  find  in  A, 
which  is  the  spectrogram  of  a  one-tenth  saturated  solution  of  neodymium 
acetate,  greater  changes  than  for  the  saturated  solution  of  the  chloride. 
This  is  in  accord  with  the  results  obtained  from  the  effect  of  dilution;  this,  it 
will  be  seen,  was  greatest  with  the  acetate.  This  tends  to  strengthen  the  view 
that  the  bands  X4275  and  X5800  are  in  some  way  associated  with  the  molecules. 

In  B  of  this  plate  there  is  given  the  spectrogram  of  the  same  solution  as 
the  temperature  was  lowered.  The  temperatures,  beginning  with  the  strip 
nearest  the  spark  spectrum,  were  190°,  165°,  145°,  125°,  100°,  75°,  50°,  25°; 
the  cell  and  arrangement  of  apparatus  were  the  same  as  in  J..  The  nega- 
tive shows  changes  the  reverse  of  those  discussed  in  section  A.  The  X4275 
band  gradually  assumes  the  sharply  defined  edges  as  the  temperature  falls, 
and  strip  8  of  5  corresponds  exactly  to  strip  loiA.  In  a  word,  there  has 
been  no  permanent  change  produced  by  heating  the  solution.  This  change 
in  the  width  of  the  absorption  bands  could  not  have  been  produced  by  any 
substance  dissolved  from  any  parts  of  the  apparatus,  as  there  is  no  reason  to 
suppose  that  this  should  disappear  as  the  solution  was  cooled.  It  seems, 
then,  that  the  broadening  is  solely  a  temperature  phenomenon. 

Plate  6  was  made  to  show  the  relative  effect  of  rise  in  temperature  on  a 
solution  of  neodymium  acetate,  as  compared  with  the  same  concentration 
of  neodymium  chloride.  The  concentration  in  each  case  was  one-tenth  sat- 
uration, the  cell  depth  being  10  cm.  The  temperatures  in  A  (neodymium 
acetate),  beginning  with  the  strip  nearest  the  numbered  scale,  were  20°, 
40°,  60°,  80°,  100°,  and  125°,  respectively.  The  temperatures  in  B  (neo- 
dymium chloride),  reading  in  the  same  order  from  the  strip  nearest  the 
spark  fines,  were  15°,  40°,  65°,  90°,  115°,  140°,  165°,  and  190°,  respectively. 
A  comparison  of  the  two  sections  of  this  plate  shows,  first,  that  for  the 
same  concentrations  of  the  two  salts  the  absorption  bands  are  wider  and 
more  pronounced  with  the  acetate  than  with  the  chloride. 

In  each  of  these  plates  only  the  X4275  and  X5800  bands  show  appreciable 


12  ABSORPTION   SPECTRA   OF   SOLUTIONS 

change  with  rise  in  temperature.  While  the  percentage  change  in  the 
former  of  these  two  bands  is  perhaps  greater,  this  shows  very  poorly  on 
the  prints  from  the  original  films.  The  X427o  band  is  very  sharp  at  the 
lower  temperatures,  but  shades  rapidly  towards  the  red  as  the  temperature 
is  raised,  the  X5800  band,  which  is  most  affected  by  temperature  changes, 
showing  decidedly  more  widening  with  the  acetate  than  with  the  chloride. 
This  is  exactly  what  we  should  expect  if  this  band  were  associated  with  the 
undissociated  molecules  of  the  salt  in  question.  The  acetate,  being  a  salt  of 
a  very  weak  acid,  is  dissociated  considerably  less  than  the  chloride,  and  con- 
sequently the  change  is  greater  in  the  case  of  the  acetate  where  there  are 
present  a  larger  number  of  molecules. 

The  facts,  then,  are :  The  number  of  molecules  in  a  given  concentration  of 
neodymium  acetate  is  greater  than  in  the  corresponding  concentration  of 
neodymium  chloride.  Hydration  decreases  with  rise  in  temperature.  The 
band  X5800  is  more  marked  in  the  acetate  than  in  the  chloride,  and  widen- 
ing with  rise  in  temperature  indicates  that  it  is  in  some  way  associated  T\dth 
the  hydrated  molecules. 

NEODYMIUM  SULPHATE  IN  WATER  AND  COBALT  CHLORIDE  IN  WATER. 

(See  Plate  7.) 

On  account  of  the  slight  solubility  of  neodymium  sulphate  in  water,  only 
the  saturated  solution  was  studied.  The  depth  of  cell  was  10  cm.  and  the 
temperatures,  beginning  with  the  strip  nearest  the  numbered  scale,  were  20°, 
45°,  75°,  90°,  115°,  and  140°,  respectively. 

It  is  seen  that  the  first  four  strips  of  A  (neodymium  sulphate)  show  the 
regular  widening  of  X4275  and  X5800.  In  strips  5  and  6  all  of  the  bands 
decrease  in  width.  This  is  especially  noticeable  in  bands  X5800,  X5100,  and 
X5225.  This  is  no  doubt  due  to  the  fact  that  some  of  the  salt  crystallized 
out  at  this  temperature,  and  the  solution  consequently  became  more  dilute. 
When  the  cell  was  opened  it  was  found  that  nearly  all  of  the  salt  had  crys- 
tallized out. 

It  is,  however,  obvious  that  the  sulphate  presents  no  exception  to  the 
general  rule  that  the  bands  widen  with  rise  in  temperature.  This  is  cer- 
tainly true  up  to  115°,  at  which  temperature  the  crystals  form  rapidly,  and 
the  effect  of  increase  in  dilution  more  than  overcomes  the  counter  effect  of 
rise  in  temperature. 

B  is  the  spectrogram  of  a  solution  of  cobalt  chloride,  1  cm.  deep  and  0.25 
normal.  The  temperatures,  beginning  with  the  strip  nearest  the  numbered 
scale,  were  12°,  32°,  52°,  76°,  92°,  112°,  132°,  and  152°.  This  plate  shows  an 
intense  absorption  in  the  violet  up  to  X3600;  also  a  broad,  hazy  band  with  its 
center  near  X5100.  On  account  of  the  haziness  of  the  cobalt  bands,  it  is 
difficult  to  discuss  them  in  detail.  The  change  produced  by  rise  in  temper- 
ature, however,  is  very  slight.  The  cobalt  salt  was  hydrolyzed  very  greatly 
at  the  higher  temperatures,  and  this  also  interfered  with  the  study  of  it^ 
absorption. 


AS  AFFECTED   BY  TEMPERATURE.  13 

PRASEODYMIUM  CHLORIDE  IN  WATER.     (See  Plate  8.) 

A  represents  the  effect  of  rise  in  temperature  on  the  absorption  spectra  of 
a  2.56  normal  solution  of  praseodymium  chloride,  the  depth  of  cell  being 
1  cm.  The  temperatures,  beginning  with  the  strip  nearest  to  the  numbered 
scale,  were  20°,  50°,  80°,  100°,  120°,  140°,  and  160°,  respectively.  The  orig- 
inal film  shows  general  transmission  from  X3400  to  X4350,  with  the  sharply 
defined  absorption  band  extending  from  X4300  to  X4750.  There  is  faint 
transmission  near  X4550.  There  is  practically  no  change  in  either  edge  of 
this  band  as  the  temperature  of  the  solution  is  raised.  There  is  a  slight 
widening  of  that  band  whose  center  is  near  X4825.  The  X5900  band  changes 
less  than  25  a.u.  over  the  entire  range  of  temperature  studied. 

B  is  the  absorption  of  a  solution  of  the  same  salt,  having  a  concentration 
of  0.256  normal  and  a  depth  of  layer  of  10  cm.  The  temperatures,  begin- 
ning with  the  strip  nearest  the  numbered  scale,  were  20°,  40°,  65°,  90°,  115°, 
140°,  165°,  and  190°,  respectively.  There  are  well-defined  bands  having 
their  centers  near  X4425,  X4650,  X4820,  and  X5900.  None  of  these  bands 
shows  any  appreciable  change  with  rise  in  temperature. 

PRASEODYMIUM  NITRATE  IN  WATER.      (See  Plate  9.) 

The  concentration  of  the  solution  used  in  making  A  was  2.6  normal;  the 
depth  of  cell,  1  cm.  The  temperatures,  beginning  with  the  strip  nearest  the 
numbered  scale,  were  12°,  32°,  52°,  72°,  92°,  112°,  125°,  and  145°,  respectively. 

In  the  ultra-violet  the  absorption  extends  to  about  X3500  in  strip  1,  but 
rapidly  increases  as  the  temperature  is  raised,  until  in  strip  8  there  is  com- 
plete absorption  as  far  as  X3800. 

There  is  a  very  intense  double  absorption  band  from  X4350  to  X4725  with 
faint  transmission  near  X4540.  This  transmission  rapidly  decreases  as  the 
temperature  is  raised,  and  entirely  disappears  at  a  temperature  slightly 
above  100°.  The  X4650  band  widens  towards  the  red  end  about  25  a.u. 
Band  X4825  shows  a  total  widening  of  about  30  A.u.  over  the  range  of  tem- 
perature studied.  The  orange  band  near  X5900  shows  a  uniform  total  wid- 
ening of  about  25  a.u.  From  this  plate  it  is  seen  that  none  of  the  praseo- 
dymium bands  shows  very  marked  change  with  rise  in  temperature;  at  this 
concentration  all  of  them  become  slightly  wider  at  the  higher  temperatures. 

In  section  B  of  this  plate  is  given  the  spectrogram  of  a  0.26  normal  solu- 
tion of  the  same  salt,  the  depth  of  the  absorbing  layer  being  10  cm.  The 
temperatures,  beginning  with  the  strip  nearest  the  numbered  scale,  were 
20°,  45°,  70°,  95°,  115°,  135°,  and  165°,  respectively.  On  this  plate,  bands 
appear  which  have  their  centers  near  X4425,  X4650,  X4825,  and  X5900;  the 
ultra-violet  absorption  bands  near  X3500.  None  of  these  bands  shows  any 
appreciable  change  over  the  range  from  20°  to  165°. 

The  plate  which  was  used  to  study  the  effect  of  dilution  upon  this  same 
salt  reveals  the  fact  that  only  in  the  most  concentrated  solutions  were  the 
bands  affected  at  all,  while  in  the  dilute  solutions  all  the  bands  remained 
unchanged.     Plate  9  shows  that  temperature  also  has  a  slight  effect  only  in 


14  ABSORPTION   SPECTRA   OF   SOLUTIONS 

the  concentrated  solutions,  while  in  the  dilute  solutions  the  bands  remain 
unchanged.  In  a  word,  rise  in  temperature  and  decrease  in  dilution  produce 
the  same  effect  upon  solutions  of  praseodymium  nitrate. 

URANYL  NITRATE  IN  WATER.     (See  Plate  10.) 

The  concentration  of  the  solution  used  in  making  A  was  0.2  normal,  the 
depth  of  layer  being  1  em.  The  temperatures,  beginning  with  the  strip 
nearest  the  spark  spectrum,  were  20°,  40°,  60°,  80°,  100°,  and  120°,  respec- 
tively. In  every  strip  the  exposure  to  the  entire  spectrum  was  made  for  30 
seconds,  a  screen  cutting  off  all  wave-lengths  beyond  X4500  was  inserted, 
and  the  ultra-violet  end  exposed  an  additional  8  minutes. 

Since  all  the  uranyl  bands  occur  in  the  violet  and  ultra-violet  end  of  the 
spectrum,  where  general  absorption  is  greatest,  due  to  precipitates  formed 
by  heating  the  solutions,  etc.,  it  was  found  very  difficult  to  obtain  satis- 
factory results.  So  far  as  this  plate  shows,  there  is  no  decided  change  in  any 
particular  band.  The  entire  series  seems  to  widen  as  the  temperature  is 
raised,  and  at  the  same  time  the  center  of  the  band  is  slightly  shifted  toward 
the  red  end  of  the  spectrum.  The  general  absorption,  ending  near  X3500  in 
strip  1,  advances  rapidly  towards  the  red  as  the  temperature  is  raised.  The 
broad  diffuse  edges  of  all  the  bands  shade  uniformly  into  each  other,  until 
at  the  highest  temperature  they  appear  as  one  broad,  hazy  absorption  band, 
extending  from  X3800  to  X4300.  At  least  a  part  of  this  is  due  to  general 
absorption. 

In  section  B  is  given  the  absorption  of  a  0.02  normal  solution  of  uranyl 
nitrate,  the  depth  of  absorbing  layer  being  10  cm.  The  red  end  of  the  spec- 
trum, beyond  X4500,  was  exposed  8  seconds,  while  the  ultra-violet  below 
X4500  had  an  exposure  of  SJ  minutes  to  the  same  source  of  light.  The  tem- 
peratures, beginning  with  the  strip  nearest  the  numbered  scale,  were  20°, 
45°,  70°,  95°,  115°,  140°,  and  165°,  respectively.  Eleven  bands  occur  between 
X3500  and  X4600.  As  the  temperature  is  raised,  all  the  bands  become  more 
diffuse  and  broader;  the  band  whose  center  is  near  X4180  seems  to  be  most 
affected.  The  red  edge  of  the  band  shades  towards  the  red  end  of  the  spec- 
trum as  much  as  25  a.u.  The  effect  produced  on  this  band  by  elevated  tem- 
peratures is  more  marked  than  in  any  of  the  other  bands.  There  is  very 
broad  and  hazy  absorption  around  X5100,  X5600,  and  X6200.  This  increases 
with  rise  in  temperature. 

It  has  been  found  very  difficult  to  give  an  exact  description  of  what  takes 
place  in  any  uranyl  band  as  the  temperature  is  raised,  since  the  edges  of  the 
bands  are  so  hazy  and  the  general  absorption  so  marked  in  the  region  of  the 
spectrum  at  which  these  bands  occur.  Only  the  general  statement  can  be 
made  that  all  uranyl  bands  become  more  diffuse  with  rise  in  temperature, 
and  in  the  band  X4165  there  is  a  decided  shading  on  the  red  edge. 
URANYL  SULPHATE  IN  WATER.     (See  Plate  XL) 

The  concentration  of  the  solution  used  in  making  A  was  0.166  normal  and 
the  depth  of  cell  1  cm.  The  respective  temperatures,  beginning  with  the 
strip  nearest  the  numbered  scale,  were  20°,  45°,  70°,  90°,  115°,  135°,  155°, 


AS  AFFECTED   BY   TEMPERATURE.  15 

and  185°.  The  part  of  the  spectrogram  above  X4550  was  exposed  40  seconds, 
while  below  that  wave-length  the  exposure  was  10  minutes.  The  apparent 
band  extending  entirely  across  the  spectrogram  near  X4550  is  the  edge  of  the 
screen  used  in  making  the  long  exposure  on  the  violet  end  of  the  spectrogram 
and  must  not  be  confused  with  an  absorption  band. 

Absorption  bands  X4175  and  X4325  have  their  centers  shifted  towards  the 
red  end  of  the  spectrum  about  25  a.u.  The  red  edges  of  bands  X4325  and 
X4550  shade  rapidly  towards  the  red.  The  well-marked  band  X4750  remains 
unchanged  throughout  the  spectrogram. 

The  encroachment  of  the  general  absorption  in  the  ultra-violet  towards 
the  red  causes  band  X3625  to  disappear  above  the  fourth  strip,  while  band 
X3750  is  scarcely  visible  above  strip  5.  All  bands  below  X4500  become  very 
diffuse  as  the  temperature  is  raised,  and  at  the  highest  temperature  are 
hardly  more  than  a  single  broad,  hazy  absorption  band  extending  from 
X4000  to  X4400. 

Section  B  is  the  spectrum  of  a  0.02  normal  solution  of  uranyl  sulphate, 
the  depth  of  absorbing  layer  being  10  cm.  The  respective  temperatures, 
beginning  with  the  strip  nearest  the  numbered  scale,  were  20°,  45°,  70°,  95°, 
115°,  140°,  and  165°.  The  exposures  were  8  seconds  in  the  visible  part  of  the 
spectrum  and  an  additional  exposure  of  4  minutes  to  the  ultra-violet.  The 
same  changes  described  in  A  take  place  here,  i,  e.,  a  strong  general  absorp- 
tion in  the  ultra-violet  beyond  X3500,  and  increasing  towards  the  red  as  the 
temperature  is  raised.  The  most  marked  widening  is  in  bands  X4100, 
X4200  and  X4350;  in  each  the  center  shifted  slightly  towards  the  red.  Such 
is  also  the  case  with  the  red  edge  of  band  X4600.  The  X4750  band  remains 
fixed  throughout  the  spectrogram.  The  very  broad,  hazy  bands  around 
X5100,  X5600,  and  X6200  appear,  and  are  not  appreciably  affected  by  changes 
in  temperature. 

URANYL  ACETATE  IN  WATER.     (See  Plate  12.) 

In  plate  12,  A  shows  the  effect  of  dilution,  B  of  temperature.  The  con- 
centrations of  the  solutions  used  in  A,  beginning  with  the  strip  farthest 
removed  from  the  numbered  scale,  were  0.25,  0.125,  0.062,  0.042,  0.0025, 
and  0.0005  normal.  So  far  as  we  can  judge  from  this  plate,  none  of  the 
absorption  bands  changes.  Beer's  law  seems  to  hold  to  the  dilution  0.0005 
normal. 

B  shows  the  effect  of  rise  in  temperature  on  a  0.02  normal  solution  of 
uranyl  acetate.  The  temperatures,  beginning  with  the  strip  nearest  the 
numbered  scale,  were  20°,  45°,  70°,  95°,  115°,  and  140°.  The  exposures  at 
that  part  of  the  spectrum  having  a  wave-length  greater  than  X4500  was  8 
seconds,  while  an  additional  exposure  of  3  minutes  was  given  to  the  ultra- 
violet end.  Every  one  of  the  nine  bands  shows  a  slight  widening  with  rise  in 
temperature.  While  in  strip  1  the  bands  are  well  marked,  they  appear 
much  more  diffuse  as  the  temperature  is  raised.  The  apparent  change  in 
the  band  near  X4475  is  probably  due  to  the  screen  used  to  cut  off  the  visible 
spectrum,  while  additional  exposure  was  made  to  the  ultra-violet  region. 


CHAPTER  III. 

EFFECT  OF  DILUTION  ON  THE  ABSORPTION  OF  LIGHT 
BY  SOLUTIONS. 

The  question  as  to  the  effect  of  dilution  on  the  power  of  solutions  to 
absorb  light  is  an  old  one.  This  question  became  especially  prominent  at 
the  time  the  theory  of  electrolytic  dissociation  was  proposed.  In  dilute 
solutions  of  electrolytes  there  are  practically  only  ions  present,  very  few 
molecules  existing  as  such.  All  of  the  properties  of  such  solutions  are  the 
properties  of  the  ions  contained  in  them.  Therefore,  the  power  of  these 
solutions  to  absorb  light  must  be  due  to  the  ions  present  in  them.  This  was 
the  reasoning  in  vogue  and  the  conclusion  drawn.  It  was  at  the  same  time 
freely  recognized  that  molecules  in  solution  have  the  power  to  absorb  light. 
This  was  shown  by  the  fact  that  solutions  of  non-electrolytes,  or  completely 
unionized  substances,  are  often  colored ;  and  color  in  solution  means  selective 
absorption  of  light. 

The  result  of  the  conclusion  drawn  from  the  theory  of  electrolytic  disso- 
ciation was  that  an  enormous  amount  of  work  was  done  on  the  absorption 
spectra  of  dilute  solutions  of  both  electrolytes  and  non-electrolytes.  Ostwald 
carried  out  an  elaborate  investigation  on  the  relation  between  color  and  dis- 
sociation, and  pubUshed  the  work  under  the  title  ''IJber  die  Farbe  der 
lonen."^  A  large  number  of  salts  were  brought  within  the  scope  of  this 
investigation — salts  of  an  acid  having  a  colored  anion,  with  colorless  cations, 
This  is  illustrated  by  the  various  permanganates,  hydrogen,  sodium,  ammo- 
nium, magnesium,  zinc,  cadmium,  etc.  Ostwald  showed  that  these  salts  of 
any  given  acid  had  essentially  the  same  spectra.  In  a  similar  manner,  he 
studied  salts  of  fluorescein,  eosin,  iodoeosin,  rosolic  acid,  diazoresorcinol,  etc. 
Ostwald  then  reversed  the  process  and  compared  the  salts  of  a  given  colored 
base  with  colorless  acids,  thus  studying  the  salts  of  p-rosaniline  with  acetic, 
chloric,  benzoic,  hydrochloric,  nitric,  butyric,  salicylic,  lactic,  etc.,  acids 
and  finding  practically  the  same  absorption  spectra  for  all  of  these  salts. 

From  the  standpoint  from  which  he  undertook  his  investigation,  Ostwald 
may  be  said  to  have  solved  the  problem  of  the  role  of  ions  in  the  absorption 
of  light,  as  far  as  that  could  be  done  with  the  prism  spectroscope. 

The  problem  that  we  studied  was  of  a  different  nature.  It  had  to  do  with 
the  absorption  spectra  of  ions  relative  to  that  of  the  molecules  from  which 
they  were  formed.  Some  earlier  work  of  Jones  and  Anderson^  had  shown 
that  if  molecules  have  different  action  on  light  from  ions,  the  difference  is  so 
slight  that  there  would  be  no  hope  of  detecting  it  by  ordinary  means,  even  with 
a  grating  spectroscope .    This  problem  was  attacked  in  the  following  manner : 


1  Zeit.  phys.  Chem.,  9,  579  (1892).  « Cam.  Inst.  Wash.  Pub.  No.  110. 

17 


18  ABSORPTION   SPECTRA   OF  SOLUTIONS. 

MAKING  A  DILUTION  SPECTROGRAM. 

Before  entering  upon  a  detailed  discussion  of  the  spectrograms,  it  is 
wise  to  state  briefly  the  method  used  in  making  any  given  spectrogram. 
Throughout  all  the  work  done  on  the  effect  of  high  dilution  on  absorption 
spectra,  under  the  conditions  of  Beer's  law,  only  three  exposures  were  made 
for  any  given  spectrogram,  i.  e.,  only  three  dilutions  were  compared.  The 
depths  of  cell  in  all  cases  were  0.5  cm.,  50  cm.,  and  250  cm.,  the  dilution  being 
increased  100  times  between  the  first  two  solutions  and  5  times  between  the 
last  two;  or  a  total  dilution  of  500  times  between  the  first  and  last  solution. 
Smaller  depths  of  cell  than  5  mm.  were  not  used,  on  account  of  the  large 
percentage  error  in  measuring  such  depths. 

Much  difficulty  was  experienced  in  getting  sufficient  light  through  the 
longer  cells  to  fill  the  grating  completely;  nor  was  this  possible  unless  the 
tube  containing  the  solution  was  constantly  moved  backwards  and  forwards 
so  that  the  image  of  the  source  of  light  was  moved  along  the  slit  of  the  camera. 
By  such  a  procedure  the  surface  of  the  grating  could  be  illuminated  fairly 
uniformily,  and  the  exposures  gave  good  results  on  the  photographic  plate, 
as  is  shown  by  the  spectrograms. 

In  order  to  insure  complete  illumination  of  the  grating  a  uniform  pro- 
cedure was  adopted.  The  longest  cell,  containing  the  most  dilute  solution, 
was  first  placed  in  position,  the  light  passed  through,  and  the  image  of  the 
Nernst  glower  sharply  focused  on  the  slit  of  the  camera  in  such  a  manner  as 
to  throw  as  much  light  as  possible  on  the  grating.  By  holding  the  eye  in  the 
position  later  to  be  occupied  by  the  photographic  plate,  we  could  easily  tell 
when  the  grating  was  properly  illuminated. 

After  everything  was  properly  adjusted  the  lights  were  extinguished  and 
the  plate  inserted  in  the  camera.  Great  care  was  taken  not  to  move  any 
parts  of  the  apparatus,  the  camera  was  closed,  the  source  of  light  again 
turned  on,  and  the  exposure  made.  It  is  clearly  seen  that  in  making  any 
spectrogram,  using  three  cells  differing  in  length  so  markedly,  we  virtually 
had  three  different  sources  of  light,  and,  consequently,  the  length  of  exposure 
sufficient  to  give  comparable  results  on  the  photographic  plate  had  to  be 
determined  by  a  long  series  of  trials.  In  the  case  of  the  longest  cell,  expos- 
ures as  long  as  several  minutes  were  made,  while  with  the  shortest  cell  only 
a  few  seconds  were  necessary  to  give  good  clear  spectrograms  on  the  photo- 
graphic plate. 

The  remaining  procedure  was  essentially  the  same  as  that  described  by 
Jones  and  Anderson^  and  by  other  workers  in  this  laboratory. 

NEODYMIUM  CHLORIDE  IN  WATER.     (See  Plate  13.) 
The  concentrations  of  the  solutions  used  in  making  the  negative  for  A^ 
beginning  with  the  one  whose  spectrum  is  farthest  from  the  spark  spectrum, 
were  2.05,  0.0205,  and  0.00401  normal,  respectively,  the  corresponding 
depths  of  absorbing  layer  being  0.5  cm.,  50  cm.,  and  250  cm. 

iCarn.  Inst.  Wash.  Pub.  110. 


EFFECT   OF  DILUTION   ON   ABSORPTION   OF   LIGHT.  19 

For  B  the  concentrations  used  were  1.025,  0.01025,  and  0.00205  normal. 
The  depths  of  layer  were  the  same  as  used  m  A.  It  is  seen  that  the  dilu- 
tions are  just  one-half  those  of  the  corresponding  layers  in  A. 
.  The  concentrations  of  solutions  used  in  making  C  were  just  half  of  those 
in  S,  i.  e.,  0.512,  0.00512,  and  0.00102  normal.  In  the  entire  plate,  as  in 
all  the  dilution  work,  the  most  dilute  solution  is  always  nearest  the  spark 
spectrum. 

Since  very  much  of  the  finer  detail  and  several  of  the  narrowest  bands 
are  lost  in  reproducing  and  printing  the  films,  our  discussion  is  always  based 
upon  the  original  photographic  film.  Lines  will  frequently  be  discussed 
which  do  not  appear  on  the  printed  plates,  but  which  are  very  clear  and  dis- 
tinct on  the  photographic  film. 

A  study  of  A  shows  complete  absorption  in  the  violet  up  to  X3350,  then 
slight  transmission  for  about  50  a.u.  The  faint  hazy  band  X3400  and  the 
well-defined  band  X3450-X3600  are  not  affected  by  the  change  in  dilution. 
Hazy  bands  appear  at  X3820,  X4040,  and  X4200.  Their  intensities  do  not 
seem  to  be  affected  by  dilution.  The  beautiful  sharp  band  X4275  is  slightly 
more  intense  in  the  most  concentrated  solution.  The  effect  of  dilution,  if 
any,  on  the  bands  X4325,  X4440,  X4600,  X4690,  X4750,  X4820  is  not  measur- 
able. On  the  original  film  they  appear  slightly  broader,  but  not  more 
intense,  on  the  third  strip. 

Bands  which  have  their  centers  near  X5100,  X5200,  and  X5800  are  decidedly 
affected  by  dilution,  the  former  two  appearing  distinctly  as  independent 
bands  in  the  most  dilute  solution,  diffuse  with  a  single  broad  band  with  the 
center  near  X5150.  There  is  the  greatest  change  between  the  second  and 
third  strips  (in  discussing  any  plate,  strip  1  is  always  nearest  the  spark  lines). 
The  broadening  of  these  bands  with  increase  in  concentration,  both  of  which 
have  rather  hazy  edges,  is  fairly  uniform,  i,  e,,  they  widen  both  towards  the 
red  and  violet  ends  of  the  spectrum. 

The  intense  band  which  extends  from  X5690  to  X5850  is  affected  very 
markedly  by  concentration,  the  widening  being  almost  entirely  towards  the 
red  end  of  the  spectrum.  The  violet  edge  is  hardly  affected,  while  the  wid- 
ening towards  the  red  is  about  50  a.u.  Here  also  the  change  in  the  width  of 
the  band  is  greatest  where  the  change  in  concentration  of  the  solution  is 
greatest.  There  is  a  very  faint  band,  X6225,  which  appears  slightly  more 
diffuse  in  the  most  concentrated  solution. 

The  concentrations  of  the  solutions  used  in  B  are  just  one-half  those  of  A, 
and  it  is  seen  that  some  of  the  smaller  bands  are  lost,  while  the  broader  ones 
have  split  into  two  or  more  smaller  bands.  In  this  film,  bands  near  X3425, 
X3475,  X3520,  X3575,  X4275,  X4340,  X4450,  X4700,  X4750,X4820,X5100,X5120, 
show  no  change  with  dilution.  The  broad  band  X5700-X5825  shows  a  widen- 
ing of  about  25  A.u. ,  being  the  only  band  which  is  changed  by  concentration. 

C  of  this  plate  is  the  spectrogram  of  solutions  twice  as  dilute  as  those  of  B. 
No  band  on  this  plate  shows  any  appreciable  change  produced  by  dilution, 
except  probably  a  slight  widening  of  X5750. 


20  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

We  then  see,  from  a  study  of  this  plate,  that  in  A  bands  X4270,  X5100, 
X5200,  and  X5750  narrow  with  dilution,  the  amount  of  change  being  in  the 
order  given;  that  is,  least  in  XolOO  and  greatest  in  X5700.  In  B  there  is  an 
appreciable  change  in  oxAy  X5750,  while  in  C  none  of  the  bands  are  affected 
by  dilution. 

NEODYMIUM  BROMIDE  IN  WATER.     (See  Plate  14.) 

The  concentrations  of  the  solutions  used  in  making  negative  A^  beginning 
with  the  solution  whose  spectrum  is  farthestfromthescale,  were  1.66, 0.0166, 
and  0.0033  normal;  the  corresponding  depths  of  absorption  layer  being  0.5 
cm.,  50  cm.,  and  250  cm.,  respectively. 

The  concentrations  used  in  making  B  were  half  of  those  of  J.,  and  those  of 
C  half  those  of  B.  The  same  range  of  cell  depth  was  used  in  all  three  sec- 
tions of  this  plate,  viz,  0.5  cm.,  50  cm.,  and  250  cm.,  respectively,  beginning 
with  the  strip  farthest  from  the  spark  lines.  In  A ,  characteristic  absorption 
bands  appear  at  X3400,  X3525,  X3800,  X4275,  X4450,  X4700,  X4750,  X4800, 
which  are  hardly  affected  by  change  in  dilution,  except  for  a  slight  increase 
in  the  intensity  of  band  X4275  in  the  most  concentrated  solution. 

The  three  bands,  X5090,  X5120,  and  X5210,  narrow  uniformly  with  dilution, 
the  greatest  change  being  between  strips  2  and  3,  where  the  change  in  dilu- 
tion is  the  greatest.  With  the  bromide,  as  is  seen  in  plate  14,  the  effect  of 
dilution  is  most  pronounced  in  band  X5750.  The  shading  is  almost  exclu- 
sively towards  the  red,  the  violet  edge  remaining  practically  unchanged. 
This  edge  shows  no  change  between  strips  1  and  2,  yet  the  red  edge  is 
widened  as  much  as  30  a.u. 

In  5,  where  the  concentrations  were  0.83,  0.0083,  and  0.00166  normal, 
respectively,  the  depths  of  absorbing  layer  were  the  same  as  used  in  A, 
There  is  no  measurable  change  in  any  of  the  absorption  bands  except  the 
band  whose  center  is  near  X4800.  This  band  shows  the  characteristic  nar- 
rowing with  dilution,  as  the  dilution  is  increased.  The  total  change  is  not 
greater  than  20  a.u.    Band  X5200  is  shghtly  more  intense  in  the  third  strip. 

When  we  reach  the  dilution  used  in  C,  which  is  four  times  that  of  J.,  any 
change  due  to  dilution  has  disappeared  except  a  narrowing  of  probably  10 
a.u.  between  the  third  and  second  strips. 

Taking  plate  14  as  a  whole  we  see,  first,  the  narrowing  due  to  increased 
dilution  is  most  marked  in  A,  less  in  B,  and  least  in  C.  This  is  seen  to  be 
the  same  order  as  their  respective  concentrations.  Considering  an  indi- 
vidual section,  we  find  the  most  pronounced  narrowing  where  the  change  in 
dilution  is  greatest — that  is,  between  strips  2  and  3. 

NEODYMIUM  NITRATE  IN  WATER.     (See  Plate  15.) 

The  concentrations  of  neodymium  nitrate  used  in  making  negative  A  of 
this  plate,  beginning  with  the  strip  farthest  from  the  numbered  scale,  were 
2.15,  0.0215,  and  0.00430  normal,  the  corresponding  depths  of  absorbing 
layer  being  0.5  cm.,  50  cm.,  and  250  cm.,  respectively. 


EFFECT   OF   DILUTION    ON    ABSORPTION    OF    LIGHT.  21 

In  discussing  the  absorption  bands  of  this,  as  well  as  other  plates  through- 
out this  paper,  we  do  not  attempt  to  give  the  exact  position  of  the  band  in 
question,  as  has  previously  been  done  by  many  workers;  but  we  simply  indi- 
cate the  position  of  the  band  by  selecting  a  wave-length  near  its  center. 
For  instance,  in  speaking  of  band  X5800,  we  mean  that  broad  band  extending 
from  X5700  to  X5850.  This  is  not  confusing  and  saves  space  and  time  in 
the  description  of  any  plate. 

This  is  probably  the  best  plate  we  have  illustrating  the  effect  caused  by 
dilution.  Bands  which  are  hardly  affected  over  the  range  of  dilution  given 
in  A  are  located  at  X3525,  X3820,  X4440,  X4620,  X4750,  X4830.  In  strip  3 
the  well-defined  band  X4275  is  more  diffuse,  though  probably  not  so  intense. 
This  is  in  keeping  with  the  behavior  of  this  same  band  as  shown  by  other 
salts  of  neodymium,  though  probably  a  little  more  marked.  There  is  faint 
transmission  at  X5100  for  about  10  a.u.  In  strip  3,  representing  the  most 
concentrated  solution,  the  bands  X5090  and  X5125  have  so  broadened  that 
they  coalesce.  The  X5220  band  widens  uniformly  towards  the  red  and 
violet  as  the  solution  becomes  more  concentrated. 

Band  X5800,  which  is  most  affected  by  dilution,  shows  a  total  change  of 
probably  as  much  as  70  a.u.,  the  shading  being  largely  towards  the  red. 
In  strips  1  and  2  the  violet  edge  is  hardly  changed,  while  in  strip  3  it  is  prob- 
ably shifted  20  a.u.  On  the  original  film  these  three  strips  show  abso- 
lutely the  same  development,  hence  are  directly  comparable. 

Section  B  represents  the  absorption  of  neodymium  nitrate.  Beginning 
with  the  strip  farthest  from  the  numbered  scale,  the  concentrations  are 
1.075,  0.01075,  and  0.00215  normal,  the  corresponding  depths  of  cell  being 
the  same  as  in  A.  In  this  section  only  a  few  bands  need  be  discussed. 
Bands  X5090  and  X5125,  which  appear  as  distinct  bands  in  strips  1  and  2, 
have  slightly  broadened  so  as  to  form  a  single  hazy  band  whose  center  is 
near  X5120.  The  X5750  band  narrows  as  much  as  40  a.u.,  almost  the  entire 
change  being  between  strips  2  and  3. 

In  C,  the  X5750  band  alone  is  noticeably  changed,  narrowing  about  20 
a.u.  from  strip  3  to  strip  2,  but  is  not  changed  in  the  last  dilution,  i.  e.,  from 
strips  2  to  1. 

NEODYMIUM  SULPHATE  IN  WATER.     (See  Plate  16.) 

A  gives  the  absorption  spectra  of  a  solution  of  neodymium  acetate.  The 
concentrations  of  the  solutions  used,  beginning  with  the  strip  farthest 
removed  from  the  numbered  scale,  were  0.5,  0.01,  and  0.002  normal.  The 
corresponding  depths  of  cell  were  1,  50,  and  250  cm. 

This  additional  plate  of  neodymium  acetate  was  made  to  study  the  effect 
of  exposure  on  the  apparent  widening  of  the  bands  with  concentration. 
Strip  2  was  more  exposed  than  strip  1,  and  strip  3  had  a  longer  exposure 
than  strip  2.  Nevertheless  the  X5800  band  has  widened  as  much  as  50  A.u. 
between  the  first  and  third  exposures.  In  view  of  the  unequal  exposures  of 
these  strips,  it  is  not  thought  advisable  to  discuss  the  other  bands.     These 


22  ABSORPTION    SPECTRA    OF   SOLUTIONS. 

results  show  that  difference  in  exposure  can  not  account  for  the  changes  in 
the  widths  of  the  bands  in  question. 

Spectrograms  B  and  C  of  this  plate  give  the  absorption  of  solutions  of 
neodymium  sulphate.  On  account  of  the  slight  solubility  of  this  salt,  we 
observe  only  slight  changes  in  any  of  its  absorption  bands. 

The  concentrations  in  B  were  0.1,  0.004,  and  0.0008  normal;  the  corre- 
sponding depths  of  cell  being  2,  50,  and  250  cm.  The  concentrations  in  C 
were  0.1,  0.001,  and  0.0002  normal,  and  the  corresponding  depths  of  cell 
were  0.5,  50,  and  250  cm. 

In  both  B  and  C,  those  bands  having  their  center  near  X3500  appear  well 
defined  and  remain  unchanged  both  in  position  and  intensity  as  the  dilution  is 
changed.  The  bandX5750  widens  with  concentration  in  A  as  much  as  25  a.u.  ; 
it  remains  practically  unchanged  in  B,  where  the  solutions  are  more  dilute. 

The  plate  brings  out  the  fact  already  mentioned,  that  only  the  more  con- 
centrated solutions  show  marked  change,  either  with  change  in  temperature 
or  with  change  in  dilution. 

NEODYMIUM  ACETATE  IN  WATER.     (See  Plate  17.) 

The  concentrations  of  solutions  used  in  making  A  of  this  plate  were  sat- 
urated, one-hundredth  saturated,  and  five-hundredth  saturated,  the  corre- 
sponding depths  of  cell  being  0.5  cm.,  50  cm.,  and  250  cm.,  respectively. 
The  most  dilute  solution  is  nearest  the  numbered  scale. 

This  plate  was  made  with  very  long  exposures,  to  see  if  the  apparent 
widening  of  the  bands  could  be  due  to  the  difference  in  the  amounts  of  light 
falling  upon  the  photographic  plate.  In  such  a  procedure  the  most  concen- 
trated solution  was  given  the  longest  exposure  and  yet  had  the  broader 
bands.  It  is  possible  to  narrow  any  given  absorption  band  by  lengthening 
the  time  of  exposure,  but  this  can  not  account  for  so  large  a  difference  as  is 
shown  by  strip  3  of  section  A.  Even  in  this  section  it  is  seen  that  the  third 
strip  has  wider  bands  than  either  of  the  other  two  strips  of  this  section,  not- 
withstanding the  fact  that  the  actual  exposure  of  the  strip  is  greater.  Thus 
we  see  that  the  difference  of  exposure  can  not  account  for  the  changes  in 
the  width  of  bands  such  as  we  have  noted. 

In  section  A  the  violet  group  of  bands  in  the  region  X3500  came  out  beau- 
tifully. Such  is  only  the  case  when  quite  a  long  exposure  is  made.  Indeed, 
in  order  to  show  these  lines  clearly,  the  exposure  must  be  long  enough  to 
destroy  those  fine,  sharp  lines  in  the  region  of  X3800  to  X4600.  Hence,  in 
this  plate  the  latter  group  of  lines  do  not  appear  distinctly,  though  traces  of 
them  can  be  seen  on  the  original  film. 

The  hazy  bands  X3300  and  X3400  appear  on  this  plate  and  remain  un- 
changed by  dilution. 

The  three  bands,  X3460,  X3500  and  X3540,  remain  perfectly  constant 
throughout  the  section.  Band  X5120,  which  appears  broad  and  diffuse, 
shows  no  change.  Band  X5210  narrows  about  10  a.u.  from  the  third  to  the 
second  strip,  and  remains  unchanged  with  the  next  dilution.     The  broad 


EFFECT   OF  DILUTION   ON   ABSORPTION   OF  LIGHT.  23 

band  X5750  narrows  about  40  a. u.  from  the  third  to  the  second  strips,  and 
about  15  A.u.  with  the  next  dilution. 

The  concentrations  used  in  B  were  again  just  half  those  in  A,  the  most 
concentrated  solution  being  one-half  saturated,  with  succeeding  dilutions  of 
100  and  5  times,  respectively;  the  depths  of  cell,  beginning  with  the  strip 
farthest  removed  from  the  scale,  were  0.5  cm.,  50  cm.  and  250  cm. 

Again,  only  bands  X5220  and  X5750  are  changed,  but  with  the  acetate  the 
change  extends  farther  with  the  more  dilute  solutions.  In  a  word,  the  nar- 
rowing of  the  bands  with  dilution  is  more  marked  in  B  and  C  than  in  the  case 
of  the  chloride,  bromide,  and  nitrate.  The  group  of  bands  near  X3500  is  not 
altered  with  dilution. 

The  concentrations  used  in  C  are  again  half  of  those  in  B,  and  the  corre- 
sponding depths  of  cell  the  same  as  used  throughout  this  plate.  Their  respec- 
tive sequence  is  the  same.  In  this  spectrogram  only  band  X5750  changes, 
and,  indeed,  this  is  the  only  salt  of  neodymium  with  which  a  change  with 
dilution  has  been  noted  in  so  dilute  a  solution.  This  band  narrows  about  20  a.u. 

Neodymium  acetate,  being  the  salt  of  a  weak  acid,  is  of  course  less  dis- 
sociated at  any  given  concentration  than  a  salt  of  a  strong  acid.  This  salt 
approaches  complete  dissociation  much  more  slowly  than  the  others  studied. 
Spectrograms  A,  B,  and  C  show  also  that  the  changes  caused  by  dilution  are 
more  marked  and  extend  into  more  dilute  solutions  than  with  the  chloride, 
bromide,  or  nitrate.  In  a  word,  the  changes  in  the  absorption  bands  due  to 
dilution  seem  to  follow  the  change  in  dissociation;  that  is,  they  are  a  direct 
function  of  the  number  of  molecules  present. 

PRASEODYMIUM  CHLORIDE  IN  WATER.     (See  Plate  18.) 

The  concentrations  of  solutions  used  in  making  A,  beginning  with  the 
strip  farthest  removed  from  the  numbered  scale,  were  2.56,  0.0256,  and 
0.00512  normal,  the  corresponding  depths  of  absorbing  layer  being  0.5  cm., 
50  cm.,  and  250  cm.,  respectively. 

The  absorption  is  complete  in  the  ultra-violet  up  to  about  X3100.  The 
bands  of  praseodymium  are  for  the  most  part  broad  and  have  well-defined, 
sharp  edges.  The  violet  edge  of  band  X4450  is  very  sharp  and  unchanged 
by  dilution,  while  the  hazy  red  edge  is  hardly  affected.  Band  X4675 narrows 
towards  the  violet  about  20  a.u.,  while  band  X4830  is  entirely  unchanged. 
The  broad  band  X5900,  with  slightly  hazy  edges,  shows  a  total  narrowing  of 
about  25  A.u. 

The  concentrations  of  B  and  C,  beginning  with  the  strips  farthest  from 
the  numbered  scale,  are  1.28,  0.0128,  and  0.00256  normal  and  0.64,  0.0064, 
and  0.00128  normal,  respectively.  The  corresponding  depths  of  absorbing 
layer  were  the  same  as  in  A.  None  of  the  bands  is  affected  by  dilution, 
and  we  may  say  that  Beer's  law  holds  very  well  for  praseod3^nium  chloride, 
except  for  bands  X4675  and  X5900  in  the  most  concentrated  solutions.  Even 
here  the  change  is  very  slight  and  not  to  be  compared  with  corresponding 
changes  with  dilution  in  salts  of  neodymium. 


24  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

PRASEODYMIUM  NITRATE  IN  WATER.     (See  Plate  19.) 

The  concentrations  of  the  solutions  used  in  A,  beginning  with  the  strip 
farthest  removed  from  the  numbered  scale,  were  2.6,  0.026,  and  0.0052  nor- 
mal; the  corresponding  depths  of  cell  being  0.5,  50,  and  250  cm. 

The  concentrations  in  B  were  just  half  of  those  in  A,  and  those  in  C  were 
half  of  those  in  B.  None  of  the  absorption  bands  shows  any  change  with 
dilution  in  either  B  or  C.  In  A  there  is  a  slight  change  in  the  X4450  and 
X4650  bands.  Each  of  these  bands  widens  about  20  a.u.  with  concentration 
over  the  range  of  concentration  studied. 

With  praseodj-mium  nitrate,  as  with,  the  chloride  previously  discussed, 
there  is  only  very  slight  change  in  the  absorption  with  change  in  dilution. 
URANYL  CHLORIDE  IN  WATER.     (See  Plate  20.) 

The  concentrations  of  solutions  used,  beginning  with  the  strip  farthest 
removed  from  the  numbered  scale,  were  1.363,  0.682,  0.341,  0.227,  0.01363, 
and  0.00272  normal;  the  corresponding  depths  of  cell  being  0.5,  1,  2,  3,  50, 
and  250  cm.  In  making  this  spectrogram,  no  additional  exposure  was  made 
in  the  ultra-violet.  The  last  fom*  strips,  one  being  nearest  the  numbered 
scale,  were  each  exposed  30  seconds  to  the  Nernst  glower.  The  first  two 
strips,  on  account  of  the  length  of  cell  used,  had  to  be  exposed  a  much  longer 
time.  In  this,  as  in  all  other  cases,  the  length  of  exposure  was  governed 
solely  by  the  time  required  to  give  a  clear  print  on  the  plate. 

There  is  complete  absorption  of  all  the  light  having  wave-lengths  shorter 
than  X4500,  well-defined  bands  with  rather  hazy  edges  appearing  near  X4700 
and  X4900.  There  is  also  rather  diffuse  absorption  near  X5500  and  X6100. 
The  two  last-named  bands  are  too  ill-defined  for  detailed  discussion. 

The  X4700  band  shows  marked  widening  with  increase  in  concentration, 
the  change  being  greater  towards  the  red  end  of  the  spectrum.  The  entire 
band  is  about  50  a.u.  The  X4900  band  shades  off  rapidly  towards  the  red 
end  of  the  spectrum,  but  not  so  much  as  the  band  X4700.  The  greatest 
change  is  between  strips  5  and  6,  i.  e.,  where  the  change  in  dilution  is  greatest. 

The  concentrations  in  B  were  just  half  of  those  in  A.  Starting  with  the 
strip  away  from  the  scale,  they  were  0.685, 0.340, 0.170, 0.1135, 0.00685,  and 
0.00136  normal.  The  depths  of  the  cell  were  the  same  as  in  A,  0.5,  1,  2,  3, 
50,  and  250  cm. 

The  changes  produced  by  dilution,  as  shown  in  B,  are  much  less  marked 
than  in  A.  Indeed,  this  would  be  expected,  since  the  concentrations  of  the 
solutions  were  less.  There  is,  however,  a  gradual  widening  of  both  X4600 
and  X4700  bands  as  the  solution  becomes  more  concentrated.  The  greatest 
change  is  in  strips  5  and  6. 

URANYL  BROMIDE  IN  WATER.     (See  Plate  21.) 

The  concentrations  of  the  solutions  used  in  making  A,  beginning  with  the 
strip  farthest  removed  from  the  numbered  scale,  were  1.365,  0.682,  0.341, 
0.227,  0.01365,  and  0.0027  normal,  the  corresponding  depths  of  layer  being 
0.5, 1,  2,  3;  50,  and  250  cm. 


EFFECT  OF  DILUTION   ON   ABSORPTION   OF  LIGHT.  25 

This  spectrogram  shows  complete  absorption  in  the  violet  to  about  X4500, 
with  a  well-defined  band  near  X4700.  The  latter  widens  uniformly  with 
increase  in  concentration.  It  is  scarcely  visible  in  strip  1,  but  becomes  about 
75  A.u.  wide  in  the  top  strip.  The  most  concentrated  solution  (strip  6) 
shows  decidedly  more  absorption  towards  the  red.  The  change  in  both  of 
these  bands  is  decidedly  the  most  pronounced  between  strips  5  and  6,  i.  e., 
where  the  percentage  change  in  dilution  is  greatest. 

B  contains  the  absorption  spectra  of  a  series  of  solutions  whose  respective 
concentrations  are  just  half  of  those  in  A,  the  corresponding  depths  of 
absorbing  layer  being  the  same  as  in  A. 

There  is  faint  transmission  near  X3800,  with  complete  absorption  of  all  of 
the  wave-lengths  from  this  region  to  X4400.  None  of  the  bands  shows  any 
change  with  dilution.  In  a  word.  Beer's  law  seems  to  hold  perfectly  for 
these  dilutions. 

URANYL  NITRATE  IN  WATER.     (See  Plate  22.) 

The  concentrations  of  A,  beginning  with  the  strip  farthest  removed  from 
the  numbered  scale,  were  1.55,  0.775,  0.387,  0.269,  0.0155,  and  0.0031  nor- 
mal, the  corresponding  depths  of  cell  being  0.5,  1,  2,  3,  50,  and  250  cm. 

In  B,  the  concentrations  were  just  half  of  those  in  A,  the  same  depths  of 
cell  being  employed.  The  negative  of  A  shows  the  bands  with  special 
clearness. 

In  strip  1  there  is  complete  absorption  of  the  violet  to  X4500.  This 
gradually  recedes  towards  the  red,  with  increase  in  concentration  amount- 
ing to  as  much  as  100  a.u.  The  X4700  band  widens  about  20  a.u.  There  is 
a  sharp  band,  X4878,  which  widens  slightly  with  increase  in  concentration. 

B  shows  faint  transmission  around  X3750,  with  broad,  intense  absorption 
to  X4350.  Absorption  bands,  which  are  unchanged  by  change  in  dilution, 
appear  at  X4550,  X4700,  and  X4850.  The  only  change  in  the  bands  on  this 
plate  is  a  slight  encroachment  on  the  red  of  the  broad  violet  absorption  in 
strip  6. 

The  results  recorded  on  this  plate  are  in  complete  accord  with  those  of 
plates  20  and  21,  which  are  the  corresponding  absorptions  of  uranyl  chloride 
and  uranyl  bromide.  A ,  in  all  three  of  these  plates,  represents  the  most  con- 
centrated solutions,  while  B  represents  half  the  concentrations  in  A.  Most 
of  the  bands  in  A  show  well-marked  widening  with  increase  in  the  concen- 
tration, while  in  B  the  change  is  scarcely  detectable. 


DESCRIPTION  OF  PLATES. 

Plate 

1.  A.  Neodymium  Chloride  in  Aqueous  Solution.     Concentration,  saturated.     Depth 

of  layer,  1  cm.  Respective  temperatures,  20°,  45°,  70°,  95°,  115°,  140°,  and 
165°,  with  lowest  temperatures  nearest  spark  lines.  Exposures  made  on 
rising  temperature. 
B.  The  same  solution  used  in  A,  with  exposures  made  as  cell  cooled.  Depth  of  layer 
and  concentration  the  same  as  in  ^i.  Temperatiues,  165°,  140°,  115°,  95°, 
70°,  respectively.     Highest  tem^peratures  nearest  spark  lines. 

2.  A.  Neodymium  Bromide  in  Aqueous  Solution.     Concentration,  1,66  normal.   Depth 

of  cell,  1  cm.     Respective  temperatures,  20°,  45°,  70°,  95°,  120°,  140°,  175°, 
190°.     Lowest  temperature  nearest  spark  spectra. 
B.  Neodymium   Bromide   in   Aqueous   Solution.     Concentration,    0.166   normal. 
Depth  of  cell,  10  cm.     Respective  temperatures,  20°,  45°,  70°,  95°,  115°, 
135°,  155°,  and  190°.     Highest  temperature  nearest  spark  spectra. 

3.  A.  Neodymium  Nitrate  in  Water.     Concentration  saturated;  depth  of  cell,  1  cm. 

The  temperatures,  beginning  with  the  strip  nearest  the  numbered  scale, 
were  15°,  40°,  65°,  115°,  140°,  and  165°,  respectively. 
B.  Neodymium  Nitrate  in  Water.     Concentration,  one-tenth  saturation,  depth  of 
cell,  10  cm.;  temperatures,  20°,  45°,  70°,  95°,  120°,  and  145°,  respectively. 
Lowest  temperature  nearest  the  numbered  scale. 

4.  A.  Neodymium  Nitrate  in  Aqueous  Solution.    Concentration,  0.1  saturated.    Depth 

of  layer,  10  cm.  Temperatures,  20°,  45°,  70°,  95°,  115°,  140°,  165°,  190°. 
Exposures  made  as  temperature  was  raised.  Lowest  temperature  nearest 
spark  lines. 
B.  The  same  solution  of  neodymium  nitrate  as  used  in  A .  Concentration  and  depth 
of  layer  identical  with  A.  Temperatures,  190°,  165°,  140°,  115°,  95°.  70°, 
45°,  and  20°.  Exposures  made  on  falling  temperatures.  Highest  tempera- 
tures nearest  spark  lines. 

5.  A.  Neodymium  Acetate  in  Water.     Concentration,  0.1  saturated.     Depth  of  cell, 

10  cm.  Trace  of  acetic  acid  added  to  prevent  precipitation.  Tempera- 
tures, 20°,  45°,  70°,  95°,  120°,  140°,  160°,  and  190°.  Exposures  made  on 
rising  temperature.  Lowest  temperature  nearest  spark  line. 
B.  Solution,  depth  of  cell  and  concentration  the  same  as  A.  Temperatures,  190°, 
165°,  145°,  125°,  100°,  75°,  50°,  and  25°.  Exposures  made  on  falling 
temperatures. 

6.  A.  Neodymium  Acetate  in  Water.     Concentration,  one-tenth  saturation;  depth  of 

absorbing  layer,  10  cm.  The  temperatures,  beginning  with  strip  nearest 
the  numbered  scale,  were  20°,  40°,  60°,  80°,  100°,  and  125°,  respectively. 
B.  Neodymium  Chloride  in  Water.  Concentration  one-tenth  saturation;  depth  of 
cell,  10  cm.  The  temperatures,  beginning  with  the  strip  nearest  the 
numbered  scale,  were  15°,  40°,  65°,  90°,  115°,  140°,  165°,  and  190°,  respec- 
tively. 

7.  A.  Neodymium  Sulphate  in  Water.   Concentration  was  saturation,  cell  depth,  10  cm. 

Temperatures,  beginning  with  the  strip  nearest  the  numbered  scale,  were 
20°,  45°,  75°,  90°,  115°,  and  140°.  ^ 

B.  Cobalt  Chloride  in  Water.  Concentration,  0.25  normal;  depth  of  cell  1  cm. 
Temperatures,  12°,  32°,  52°,  76°,  92°,  112°,  132°,  and  152°,  respectively. 
The  lowest  temperature  was  nearest  the  numbered  scale. 

8.  ^.  Praseodymium   Chloride  in   Water.     Concentration,   2.56  normal;  depth    of 

absorbing  layer  1  cm.  Temperatures,  beginning  with  strip  nearest  the 
numbered  scale,  were  20°,  50°,  80°,  100°,  120°,  140°,  and  160°,  respectively. 
B.  Praseodymium  chloride  in  water.  Concentration,  0.256  normal;  depth  of  cell, 
10  cm.  Beginning  with  the  strip  nearest  numbered  scale,  the  temperatures 
were  20°,  40°,  65°,  90°,  115°,  140°,  165°,  and  190°,  respectively. 

9.  A.  Praseodymium  Nitrate  in  Water.     Concentration,  2.6  normal.     Cell  depth,  1  cm. 

Temperatures,   12°,  32°,  52°,  72°,  92°,   112°,   125°,  and  145°.     Lowest 
temperature  nearest  spark  lines. 
B.  Praseodymium  Nitrate  in  Water.     Concentration,  0.26  normal.     Cell  depth, 
10  cm.     Temperatures,  20°,  45°,  70°,  95°,  115°,  135°,  and  165°.     Lowest 
temperature  nearest  spark  lines, 
26 


DESCRIPTION   OP  PLATES.  27 

10.  A.  Uranyl  Nitrate  in   Wulur.     Concentration,  0.2  normal.     Cell  depth,   1   cm. 

Temperatures,  starting  with  strip  nearest  spark  lines,  were  20",  40°,  60°, 
80°,  100°,  and  120°,  respectively. 
B.  Uranyl  Nitrate  in  Water.     Concentration,  0.02  normal.     Depth  of  cell,  10  cm. 
Temperatures,  starting  with  exposure  nearest  spark  hnes,  were  20°,  45°, 
70°,  95°,  115°,  140°,  and  165°,  respectively. 

11.  yl.  Uranyl  Sulphate  in  Water.     Concentration,  0.166  normal.     Depth  of  cell,  1  cm. 

Temperatures,  beginning  nearest  spark  Hnes,  20°,  45°,  70°,  90°,  115°,  135°, 
155°,  and  185°,  respectively. 
B.  Uranyl  Sulphate  in  Water.     Concentration,  0.02  normal.     Cell  depth,  10  cm. 
Respective  temperatures,  beginning  nearest  spark  lines,  were  20°,  45°,  70°, 
95°,  115°,  140°,  and  165°. 

12.  A.  Uranyl  Acetate  in  Water.     The  concentrations,  beginning  with  the  strip  most 

removed  from  the  numbered  scale,  were  0.25,  0.125,  0.062,  0.042,  0.0025, 
0.0005  normal,  respectively;  the  corresponding  depths  of  absorbing  layer 
were  0.5,  1,  2,  3,  50,  and  250  cm. 
B.  Uranyl  Acetate  in  Water.  Concentration,  0.02  normal.  Depth  of  cell,  10  cm. 
The  temperatures,  beginning  with  the  strip  nearest  the  numbered  scale, 
were  20°,  45°,  70°,  95°,  115°,  and  140°,  respectively. 

13.  A.  Neodymium  Chloride  in  Water.     Concentrations,  2.05,  0.0205,  and  0.00401  nor- 

mal.    Respective  depths  of  cell,  0.5,  50,  and  250  cm.     Most  dilute  solution 
nearest  spark  lines. 

B.  Neodymium  Chloride  in  Water.     Concentrations,  1.025,  0.01025,  and  0.00205 

normal.     Depths  of  cell,  starting  with  strip  farthest  from  spark  lines, 
were  0.5,  50  and  250  cm.,  respectively. 

C.  Neodymium  Chloride  in  Water.  Concentrations,  0.512, 0.00512,  and  0.00102  nor- 

mal.    Depths  of  cell,  beginning  with  strip  farthest  removed  from  spark 
lines,  were  0.5,  50,  and  250  cm.,  respectively. 

14.  A .  Neodymium  Bromide  in  Water.     Concentrations,  1.66,  0.0166,  and  0.0033  normal. 

Corresponding  depths  of  ceU,  0.5,  50,  and  250  cm.,  respectively.     Most 
dilute  solution  nearest  spark  lines. 

B.  Neodymium  Bromide  in  Water.     Concentrations,  0.83,  0.0083,  and  0.00166 

normal.     Corresponding  depths  of  cell,  0.5,  50,  and  250  cm.,  respectively. 
Most  dilute  solution  nearest  spark  Unes. 

C.  Neodymium  Bromide  in  Water.     Concentrations,  0.415,  0.00415,  and  0.000803 

normal.     Corresponding  depths  of  cell,  0.5,  50,  and  250  cm.,  respectively. 

15.  A .  Neodymium  Nitrate  in  Water.    Concentrations,  2.15, 0.0215,  and  0.00430  normal, 

respectively.     Corresponding  depths  of  cell  were  0.5,  50,  and  250  cm., 
respectively.     Most  dilute  solutions  nearest  spark  lines. 

B.  Neodymium  Nitrate  in  Water.     Concentrations,  1.075,  0.01075,  and  0.00215 

normal.     Corresponding  cell  depths  were  0.5,  50,  and  250  cm.,  respectively. 

C.  Neodymium  Nitrate  in  Water.     Concentrations,  0.537,  0.00537,  and  0.00107. 

Corresponding  depths  of  cell  were  0.5, 50,  and  250  cm.,  respectively.    Most 
dilute  solutions  nearest  spark  hnes. 

16.  A.  Neodymium  Acetate  in  Water.     Concentrations,  0.5,  0.01,  and  0.002  normal; 

the  corresponding  depths  of  absorbing  layers  being  0.1,  50,  and  250  cm., 
respectively. 

B.  Neodymium «Sulphate  in  Water.     Concentrations  0.1,  0.004,  and  0.0008  normal, 

the  corresponding  depths  of  cell  being  2,  50,  and  250  cm.,  respectively. 

C.  Neodymium  Sulphate  in  Water.     Concentrations,  0.1,  0.001,  and  0.0002  normal. 

Depths  of  cell,  0.5,  50,  and  250  cm.,  respectively.     In  each  case,  the  most 
dilute  solution  is  nearest  the  numbered  scale.  . 

17.  A.  Neodymium  Acetate  in  Water.     Concentrations,  saturated,  '0.01s  and  0.002», 

where  s  represents  a  saturated  solution  of  the  salt  in  water  at  25°.     Corre- 
sponding depths  of  cell  were  0.5,  50,  and  250  cm.,  respectively. 

B.  Neodymium  Acetate  in  Water.   Concentrations,  0.50s,  0.005s,  and  0.001s  (s  being 

a  saturated  solution  as  above).     Corresponding  depths  of  cell  were  0.5, 
50,  and  250  cm. 

C.  Neodymium  Acetate  in  Water.    Concentrations,  0.25s,  0.0025s,  and  0.0005s. 

Corresponding  depths  of  cell  were  0.5,  50,  and  250  cm.,  respectively. 

18.  A.  Praseodymium  Chloride  in  Water.    Concentrations,  2.56,  0.0256,  and  0.00512 

normal,  respectively.  Corresponding  depths  of  cell  were  0.5,  50,  and  250  cm. 

B.  Praseodymium  Chloride  in  Water.   Concentrations,  1 .28, 0.0128,  and  0.00256  nor- 

mal.   Corresponding  depths  of  cell  were  0.5,  50,  and  250  cm.,  respectively. 

C.  Praseodymium  Chloride  in  Water.   Concentrations,  0.64,  0.0064,  and  0.00128  nor- 

mal.   Corresponding  depths  of  cell  were  0.5,  50,  and  250  cm.,  respectively. 


28  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

19.  A.  Praseodymium  Nitrate  in  Water.     Concentrations,  2.G,  0.026,  and  0.0052  normal, 

respactively.     Corresponding  depths  of  cell,  0.5,  50,  and  250  cm.,  respec- 
tively; most  dilute  solution  nearest  numbered  scale. 

B.  Praseodymium  Nitrate  in  Water.     Concentration,  1.3,  0.013,  and  0.0026  normal; 

cell  depths,  0.5,  50,  and  250  cm.,  respectively. 

C.  Praseodymium  Nitrate  in  Water.     Concentration,"^  0.65,  0.0065,  and  0.0013  nor- 

mal; cell  depths,  0.5,  50,  and  250  cm.     In  each  case  the  most  dilute  solu- 
tion is  nearest  the  numbered  scale. 

20.  A.  Uranyl  Chloride  in  Water.     Concentrations,  1.363,  0.682,  0.341,  0.227,  0.01363, 

and  0.00272  normal      Depths  of  cell,  0.5,  1,  2,  3,  50,  and  250  cm.,  respec- 
tively. 
B.  Uranyl  Chloride  in  Water.     Concentrations,  0.685,  0.340,  0.170,  0.1135,  0.00685 
and  0.00136  normal,  corresponding  depths  of  absorbing  layers  being  0.5, 
1,  2,  3.  50,  and  250  cm.     The  most  dilute  solution  in  each  case  is  nearest 

21.  A.  Uranyl  Bromide  in  Water.     Concentrations,  1.365,  0.682,  0.341,  0.227,  0.01365, 

and  0.00273  normal.     Corresponding  depths  of  cell,  0.5,  1,  2,  3,  50,  and 
250  cm.     Most  dilute  solution  nearest  scale. 
B.  Uranyl  Bromide  in  Water.     Concentrations,  0.682,  0.341,  0.171,  0.113,  0.00682, 
and  0.00136  normal.     Corresponding  depths  of  cell,  0.5,  1,  2,  3,  50,  and 
250  cm. 

22.  A.  Uranyl  Nitrate  in  Water.     Concentrations,  1.55,  0.775,  0.387,  0.269,  0.0155,  and 

0.0031  normal.     Corresponding  depths  of  cell,  0.5,  1,  2,  3,  50,  and  250  cm., 
respectively. 
B.  Uranyl  Nitrate  in  Water.     Concentrations,  0.775,  0.387,  0.193,  0.134,  0.00775, 
and  0.0015  normal,  respectively.     Corresponding  depths  of  cell,  0.5,  1,  2, 
3,  50,  and  250  cm.,  respectively. 


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CHAPTER  IV. 

ABSORPTION  SPECTRA  OF  AQUEOUS  SOLUTIONS  OF  CERTAIN 

SALTS  OF  NEODYMIUM  AS  STUDIED  BY  MEANS  OF 

THE  RADIOMICROMETER. 

The  radiomicrometer  is  simply  a  thermo-element  attached  to  a  loop  of 
thin  copper  wire  suspended  in  a  magnetic  field.  One  of  the  greatest  diffi- 
culties in  constructing  this  element  is  to  obtain  copper  wire  free  from  all 
magnetic  metals.  If  perfectly  pure  copper  wire  could  be  found,  an  instru- 
ment could  be  constructed  of  almost  any  desired  sensibility. 

A  very  good  specimen  of  small  copper  wire  was  furnished  us  by  Leeds  and 
Northrup,  of  Philadelphia.  This  wire  was  dipped  in  dilute  nitric  acid  and 
the  exterior  dissolved  away  until  the  ^vire  was  of  proper  size.  The  removal  of 
the  outside  coating  of  the  wire  removed  practically  all  of  the  magnetic  mate- 
rial from  it,  this  material  probably  being  iron  from  the  dies  through  which 
the  wire  was  drawn. 

It  was  not  a  simple  matter  to  construct  a  satisfactory  thermo-electric 
junction.  The  alloys  used  in  making  this  junction  were  90  parts  bismuth 
and  10  parts  tin,  and  97  parts  bismuth  and  3  parts  antimony.  The  method 
of  making  the  thermo-electric  junction  and  of  soldering  it  on  to  the  ends  of 
the  loop  of  copper  wire  we  owe  to  Professor  A.  H.  Pfund.^  Fine  strips  of 
the  alloys  were  obtained  in  the  following  manner: 

A  few  grams  of  the  alloy  in  question  were  fused  in  a  vessel  free  from  all 
magnetic  material,  and  then  thrown  tangentially  upon  a  clean  and  smooth 
glass  plate.  In  this  way  strips  of  the  metal  were  obtained  of  almost  any 
desired  thickness.  Some  were  too  thin  to  handle,  those  used  being  about 
1  mm.  wide,  0.01  mm.  thick,  and  about  5  mm.  in  length. 

The  thermo-element  was  made  by  soldering  an  end  of  a  strip  of  one  of  the 
above-named  alloys  to  an  end  of  a  strip  of  the  other,  the  whole  having  the 
form  of  a  letter  V.  The  two  free  ends  of  the  V  were  soldered  to  the  two  ends 
of  the  loop  of  copper  wire.  The  soldered  surfaces  were  blackened  to  absorb 
the  energy  more  completely.  At  the  end  of  the  loop  of  copper  wire  opposite 
the  thermo-element  a  light  glass  rod  is  fastened.  This  carries  the  mirror 
and  is  suspended  from  above  by  a  quartz  fiber.  The  mirror  employed  was 
about  4  sq.  mm.  This  entire  system,  consisting  of  thermo-element,  loop  of 
copper  wire,  and  mirror,  weighed  about  20  mg.  It  was  suspended  by  means 
of  a  quartz  fiber  so  that  the  loop  hung  between  the  poles  of  a  strong  magnet. 


Phys.  Rev.,  34,  228  (1912).     Phys.  Zeit.,  13,  870  (1912). 


30      ABSORPTION   SPECTRA   OF  AQUEOUS   SOLUTIONS   OF   CERTAIN   SALTS 


This  entire  system  was  suspended  in  the  interior  of  a  glass  tube,  the  tube 
being  closed  by  a  ground-glass  stopper,  and  provided  with  suitable  windows 
for  exposing  the  junction  and  observing  the  mirror.  The  upright  tube  was 
provided  with  a  side  tube  for  evacuation,  and  by  a  method  devised  by  Pro- 
fessor Pfund  a  very  high  vacuum  could  be  obtained  and  maintained  for  any 
desired  length  of  time.  By  suitably  turning  the  ground-glass  stopper  in  the 
top  of  the  glass  tube,  the  loop  of  copper  wire,  mirror,  and,  indeed,  the  whole 
system,  could  be  made  to  occupy  any  position  relative  to  the  magnets,  even 
after  the  entire  system  had  been  evacuated.  The  whole  apparatus  was 
supported  upon  a  leveling  stand  and  packed  in  cotton  to  protect  it  from 
external  radiation,  the  thermal  junction  alone  being  exposed  to  the  radia- 
tion in  question. 

The  sensibility  of  the  instrument  used  can  be  seen  from  the  following 
data:  It  had  a  full  period  of  8  seconds,  and  with  a  candle  at  a  distance  of  a 
meter  gave  a  deflection  of  15  cm.  when  the  light  was  allowed  to  fall  on  the 
junction  after  passing  through  a  glass  window. 

When  the  apparatus  was  pumped  out  and  the  radiomicrometer  thus  sus- 
pended in  a  vacuum,  the  deflection  for  a  candle  at  a  distance  of  a  meter  was 
50  cm.  Since  glass  absorbs  just  about  half  the  energy  emitted  by  a  candle, 
our  radiomicrometer,  when  provided  with  a  rock-salt  window  and  exposed 
to  a  candle  at  a  distance  of  a  meter,  would  give  a  deflection  of  about  100  cm. 

How  our  instrument  compared  with  the  radiomicrometers  constructed 
and  used  by  other  investigators  can  be  seen  from  the  following  table,  taken 
in  part  from  the  paper  by  Coblentz '} 

Table  1. 


Investigator. 

Whole 
period  in 
seconds. 

Deflection  in  cm.  per  sq.  mm.; 
candle  1  m.  distance. 

Boys,  Phil.  Trans.  (A)  180, 159  (1889)  . . . 

Paschen,  Wied.  Ann.,  48,  275  (1893) 

Lewis,  Astrophys.  Joum.,  2,  1  (1895)  . . . 
Coblentz,  Bull.  Bur.  Stand.,  (1  Sept.  1907) 
Coblentz,  Bull.  Bur.  Stand.,  (1  Sept.  1907) 
Jones  and  Guy 

10 

40 
20 
40 
25 

8 

7 
7 

0.9 

3.0 

1.3 

3.6 

6.0 

8.0 

25.0  (vacuo) 
50.0  (vacuo)  rock-salt  window. 

Jones  and  Guy 

Jones  and  Guy 

The  magnetic  control  due  to  small  amounts  of  magnetic  impurities  in  the 
copper  wire  was,  of  course,  greater  the  more  sensitive  the  instrument.  For 
this  reason  the  radiomicrometer  was  not  used  in  a  vacuum.  The  length  of 
the  quartz  fiber  was  so  chosen  that  a  candle  at  a  distance  of  a  meter  gave  a 
deflection  of  16  cm.  The  half  period  was  4  seconds.  This  sensibility  was 
found  to  be  quite  sufficient  for  work  in  the  red  and  infra-red,  and  even  for 
wave-lengths  as  short  as  4,500  a.u.  The  measurement  could  be  carried 
out  quickly  and  the  magnetic  disturbance  was  practically  negligible. 

» Bull.lBur.  Standards  4,  No.  3. 


AS   STUDIED   BY  MEANS   OF   THE   RADIOMICROMETER.  31 

When  the  thermo-j  unction  was  exposed  to  the  radiation  and  the  source  of 
energy  removed,  the  instrument  returned  to  its  original  zero  position  to 
within  0.5  mm.  In  most  cases  several  readings  were  made  for  a  given 
amount  of  radiation,  and  these  usually  agreed  to  within  1  per  cent.  The 
source  of  energy  was  a  Nernst  glower  attached  to  a  storage  battery,  the 
amperage  being  1.2  and  the  voltage  110.  This  was  found  to  be  very  con- 
stant, successive  readings  in  the  same  position  of  the  spectra  agreeing  well 
with  one  another. 

The  vessels  used  for  holding  the  solutions  were  made  of  brass  and  gold 
plated.  They  were  about  4  cm.  in  diameter  and  of  the  desired  thickness. 
The  ends  were  made  of  the  best  optical  glass.  Vessels  of  the  same  thickness 
gave  practically  the  same  deflection  both  when  empty  and  when  filled  with 
water. 

METHOD  OF  PROCEDURE. 

The  light  from  a  Nemst  glower  was  rendered  parallel  by  a  lens,  then 
passed  through  the  vessel  containing  the  solution,  and  allowed  to  fall  on  the 
slit  of  a  Hilger  spectroscope.  The  solution  was  first  inserted  into  the  path  of 
the  light,  and  then  the  pure  solvent,  this  being  done  without  disturbing  the 
adjustment.  By  means  of  a  movable  framework,  first  the  vessel  containing 
the  solution  and  then  that  containing  the  solvent  were  interposed  in  the  path 
of  the  beam.  A  metal  screen  interposed  between  the  Nernst  glower  and  the 
vessel  containing  the  solution  allowed  the  light  to  pass  through  the  solution 
only  when  an  observation  was  being  made.  By  this  means  the  thermo- 
electric junction  was  exposed  to  the  radiation  only  long  enough  to  read  the 
deflection  of  the  mirror. 

The  light,  after  passing  through  the  solution  and  the  slit  of  the  spectro- 
scope, fell  upon  the  prism  of  the  Hilger  spectroscope.  A  second  slit  was 
inserted  in  the  spectroscope  instead  of  the  eye-piece.  The  light  passed  from 
the  prism  through  this  second  slit,  and  was  then  focused  on  the  thermal 
junction  of  the  radiomicrometer. 

The  Hilger  spectroscope  contained  a  milled  head,  graduated  so  that  the 
wave-lengths  could  be  read  off  directly.  By  suitably  turning  this  head 
any  desired  wave-length  could  be  thrown  upon  the  junction  of  the  radio- 
micrometer. 

The  width  of  the  slit  used  in  the  visible  part  of  the  spectrum  was  0.4  mm. 
In  the  infra-red,  where  there  is  far  more  energy,  the  slit  width  was  cut  down 
to  0.22  mm.  A  series  of  readings  was  carried  out  as  follows:  The  vessel 
containing  the  solvent  was  first  placed  in  the  path  of  the  beam  of  light,  the 
screen  removed,  and  the  deflection  of  the  mirror  noted.  Then  the  vessel 
containing  the  solution  was  put  in  the  same  place  that  was  formerly  occupied 
by  the  vessel  contaming  the  solvent,  the  screen  removed,  and  the  deflection 
again  noted.  The  prism  was  then  turned  slightly  by  means  of  the  gradu- 
ated and  calibrated  head,  and  a  new  wave-length  allowed  to  fall  on  the  jimc- 


32      ABSORPTION   SPECTRA   OF  AQUEOUS   SOLUTIONS   OF   CERTAIN   SALTS 

tion.  By  repeating  this  procedure  any  wave-length  could  be  allowed  to  fall 
on  the  junction.  If  we  represent  by  I  the  deflection  with  the  solution  in  the 
path  of  the  beam  of  light,  and  by/o  the  deflection  with  the  solvent  in  the  path 
of  the  light  beam,  the  percentage  of  light  which  passed  through  the  solution 
would  be  represented  by  I/Iq.    In  tables  2  to  5  we  have  the  ratio  of  I/Iq. 


Table  2. — Observed  Transmission  of  Neodijmium  Chloride  Solutions. 

X 

N.-3.43. 

N.=3.43. 

N.  =0.857. 

N. =0.427. 

X 

N.=3.43. 

N.=3.43. 

N. =0.857. 

N. =0.427. 

D.=2.5mm. 

D.=5mm. 

D.=10mm. 

D.=20mm. 

D.=2.5mm. 

D.=5mm. 

D.=10mm. 

D.=20mm. 

486 

72 

67 

80 

70 

676 

71 

62 

63 

57 

492 

83 

72 

82 

74 

678 

78 

67 

61 

495 

67 

61 

61 

66 

681 

85 

75 

74 

76 

499 

38 

25 

40 

41 

685 

89 

91 

97 

89 

501 

26 

13 

29 

30 

691 

92 

90 

99 

90 

503 

15 

8 

13 

16 

699 

88 

74 

97 

89 

505 

9 

2 

7 

12 

706 

65 

37 

91 

84 

506 

10 

1 

11 

11 

710 

49 

20 

80 

75 

509 

13 

2 

13 

14 

714 

33 

8 

47 

57 

513 

13 

4 

10 

17 

719 

18 

1 

29 

27 

515 

10 

6 

2 

8 

722 

7 

1 

8 

5 

516 

6 

2 

4 

5 

724 

4 

0 

1 

0 

518 

10 

4 

13 

14 

729 

6 

3 

0 

0 

520 

23 

11 

31 

29 

733 

14 

14 

0 

0 

522 

36 

18 

45 

42 

737 

26 

26 

5 

3 

525 

52 

23 

62 

56 

741 

38 

41 

17 

16 

530 

83 

78 

92 

84 

745 

50 

51 

42 

36 

535 

96 

95 

94 

89 

750 

57 

51 

64 

59 

544 

92 

95 

91 

87 

755 

59 

41 

79 

75 

550 

97 

98 

80 

79 

760 

54 

32 

84 

79 

556 

69 

59 

69 

70 

765 

45 

16 

75 

73 

559 

56 

41 

50 

53 

769 

33 

6 

57 

55 

563 

36 

22 

26 

33 

772 

21 

0 

34 

30 

565 

20 

7 

9 

12 

776 

12 

0 

14 

15 

567 

7 

4 

1 

2 

781 

9 

0 

4 

4 

572 

2 

0 

0 

0 

786 

10 

4 

0 

0 

577 

1 

1 

1 

1 

792 

17 

16 

2 

0 

579 

2 

2 

3 

2 

798 

28 

29 

6 

29 

583 

6 

2 

11 

9 

802 

44 

48 

16 

12 

585 

12 

3 

23 

20 

806 

54 

64 

34 

29 

587 

27 

8 

42 

37 

811 

63 

78 

59 

51 

589 

41 

26 

63 

55 

818 

76 

83 

79 

75 

592 

59 

45 

79 

69 

823 

78 

81 

91 

87 

595 

75 

62 

90 

81 

833 

70 

52 

95 

91 

597 

85 

77 

92 

86 

836 

61 

34 

90 

89 

600 

91 

90 

95 

89 

840 

51 

23 

80 

80 

609 

93 

92 

94 

88 

845 

45 

14 

62 

64 

611 

93 

92 

92 

87 

850 

37 

11 

37 

39 

614 

92 

90 

93 

87 

855 

34 

9 

20 

18 

622 

90 

79 

91 

82 

860 

33 

12 

16 

13 

629 

95 

88 

94 

84 

866 

38 

26 

23 

26 

638 

96 

99 

98 

91 

872 

42 

37 

41 

38 

643 

96 

99 

98 

92 

878 

59 

53 

43 

46 

650 

97 

99 

99 

92 

883 

67 

73 

54 

51 

657 

86 

82 

99 

90 

889 

73 

83 

65 

59 

660 

77 

60 

95 

87 

894 

81 

90 

76 

65 

662 

70 

49 

91 

80 

900 

92 

98 

90 

87 

666 

62 

37 

75 

68 

927 

96 

100 

100 

96 

670 

62 

36 

60 

54 

958 

100 

100 

100 

100 

672 

62 

60 

57 

48 

AS   STUDIED   BY  MEANS   OF  THE  KADIOMICROMETER.  33 

Table  3. — Observed  Transmissions  of  Neodymium  Bromide  Solutions. 


X 

N.  =  1.68. 

N.  =  1.66. 

N. =0.415. 

N. =0.208. 

X 

N.  =  1.66. 

N.  =  1.66. 

N.=0.415. 

N. =0.208. 

D.=2.5mm. 

D.=5mm. 

D.=10mm. 

D.=20mm. 

D.=2.5mm. 

D.=5mm. 

D.  =  10mm. 

D.=20mm. 

486 

64 

44 

84 

83 

672 

71 

52 

69 

67 

492 

64 

46 

81 

84 

676 

67 

57 

68 

70 

495 

60 

35 

73 

76 

678 

67 

73 

75 

76 

499 

38 

22 

54 

53 

681 

71 

77 

83 

80 

501 

29 

11 

45 

43 

685 

82 

89 

87 

90 

503 

2 

9 

33 

26 

691 

95 

92 

92 

93 

505 

17 

7 

21 

25 

699 

82 

80 

95 

92 

507 

15 

4 

26 

25 

706 

72 

60 

93 

91 

509 

17 

9 

30 

29 

710 

55 

45 

88 

83 

513 

18 

6 

28 

25 

714 

38 

28 

72 

67 

515 

13 

6 

21 

14 

719 

21 

11 

48 

39 

516 

9 

4 

11 

10 

722 

12 

6 

19 

8 

518 

16 

7 

28 

25 

724 

7 

2 

6 

1 

520 

28 

12 

39 

41 

729 

10 

6 

1 

0 

522 

43 

19 

55 

51 

733 

19 

11 

2 

1 

525 

53 

32 

66 

73 

737 

32 

21 

8 

12 

530 

71 

54 

89 

86 

741 

45 

30 

26 

25 

535 

79 

57 

100 

93 

745 

59 

40 

49 

49 

544 

84 

65 

100 

94 

750 

67 

49 

73 

69 

555 

77 

48 

95 

93 

755 

67 

50 

92 

80 

556 

69 

47 

86 

77 

760 

62 

47 

100 

83 

559 

57 

37 

73 

62 

765 

51 

37 

95 

79 

563 

42 

23 

51 

39 

769 

40 

25 

82 

66 

565 

21 

12 

26 

17 

772 

28 

16 

59 

44 

567 

9 

3 

10 

5 

776 

18 

9 

37 

24 

572 

3 

2 

2 

1 

781 

18 

6 

17 

8 

577 

3 

1 

3 

3 

786 

15 

7 

5 

1 

579 

6 

2 

9 

10 

792 

21 

12 

3 

1 

583 

6 

5 

17 

21 

798 

33 

21 

10 

9 

585 

24 

11 

29 

36 

802 

46 

27 

23 

24 

587 

39 

23 

47 

51 

806 

57 

35 

44 

42 

589 

55 

34 

63 

67 

811 

69 

45 

64 

63 

592 

69 

63 

72 

77 

818 

76 

62 

83 

82 

595 

75 

61 

86 

81 

823 

80 

66 

98 

91 

597 

78 

65 

91 

89 

833 

77 

58 

94 

94 

600 

83 

73 

93 

95 

836 

68 

53 

91 

91 

605 

87 

73 

92 

94 

840 

58 

44 

82 

82 

614 

86 

74 

94 

92 

845 

50 

34 

69 

70 

622 

88 

76 

85 

84 

850 

47 

28 

48 

46 

629 

88 

78 

90 

86 

855 

43 

24 

33 

25 

638 

91 

83 

92 

93 

866 

47 

27 

36 

38 

643 

92 

98 

94 

94 

872 

53 

34 

48 

52 

650 

91 

90 

94 

92 

878 

61 

42 

56 

60 

657 

88 

83 

93 

94 

883 

69 

48 

63 

64 

660 

81 

72 

90 

91 

889 

75 

67 

70 

71 

662 

79 

62 

87 

85 

900 

87 

69 

87 

92 

666 

74 

66 

78 

78 

958 

97 

84 

98 

100 

670 

68 

50 

70 

66 

34      ABSOKPTION   SPECTRA   OF  AQUEOUS   SOLUTIONS  OF   CERTAIN   SALTS 
Table  4. — Observed  Transmissions  of  Neodymium  Nitrate  Solutions. 


X 

N.=2.95. 

N.=2.95. 

N. =0.736. 

N.=0.368. 

X 

N.=2.95. 

N.=2.95. 

N.  =0.736. 

N.=0.368. 

D.=2.5niin. 

D.=5mm. 

D.=10mm. 

D.=20mm. 

D.=2.5mm 

D.=5TnTn. 

D.=10mm 

D.=20mm. 

486 
492 
495 
499 

80 

71 
61 
39 

95 

93 

74 
49 

80 
76 
60 

45 

672 
676 
678 
681 

67 
75 

78 
85 

53 
51 
71 

77 

61 
60 
65 

76 

61 
61 
64 

77 

"  "25" 

501 

16 

23 

38 

37 

685 

88 

82 

86 

84 

503 

11 

14 

25 

17 

691 

88 

87 

92 

90 

505 

4 

7 

25 

7 

699 

81 

78 

94 

91 

507 

9 

4 

19 

11 

706 

64 

51 

86 

86 

509 

17 

4 

23 

14 

710 

36 

33 

80 

78 

513 

11 

4 

21 

14 

714 

26 

16 

65 

65 

515 

13 

2 

16 

6 

719 

11 

8 

35 

39 

516 

7 

3 

10 

4 

722 

6 

2  . 

14 

10 

518 

13 

4 

12 

8 

724 

4 

1 

2 

3 

520 

28 

5 

27 

26 

729 

8 

1 

0 

0 

522 

41 

7 

43 

37 

733 

17 

4 

0 

0 

525 

53 

15 

55 

54 

737 

33 

11 

3 

3 

530 

83 

39 

83 

79 

741 

42 

21 

11 

12 

535 

86 

72 

91 

92 

745 

52 

25 

29 

30 

544 

87 

85 

88 

87 

750 

56 

40 

50 

50 

555 

75 

82 

78 

78 

755 

56 

42 

68 

66 

556 

58 

67 

69 

71 

760 

45 

34 

78 

75 

559 

35 

54 

54 

55 

765 

35 

26 

76 

74 

563 

15 

38 

32 

35 

769 

23 

15 

61 

61 

565 

3 

25 

14 

16 

772 

13 

7 

40 

39 

567 

1 

12 

5 

3 

776 

8 

3 

20 

23 

572 

0 

6 

2 

0 

781 

7 

2 

8 

15 

577 

0 

2 

3 

0 

786 

12 

3 

1 

2 

579 

3 

2 

3 

1 

792 

19 

7 

1 

1 

583 

4 

0 

7 

6 

798 

33 

14 

3 

5 

585 

12 

1 

13 

16 

802 

48 

26 

12 

11 

687 

26 

1 

27 

26 

806 

59 

41 

27 

27 

589 

44 

7 

43 

44 

811 

61 

54 

47 

46 

592 

64 

14 

62 

62 

818 

75 

70 

71 

65 

595 

81 

27 

75 

77 

823 

74 

74 

88 

81 

597 

89 

42 

84 

85 

833 

63 

64 

98 

92 

600 

92 

58 

89 

92 

836 

52 

54 

98 

89 

605 

98 

82 

88 

91 

840 

42 

40 

82 

75 

614 

85 

87 

88 

91 

845 

39 

29 

76 

69 

622 

85 

86 

87 

84 

850 

36 

22 

48 

46 

629 

86 

85 

91 

86 

855 

39 

18 

28 

23 

638 
643 

89 
94 

97 
98 

94 
96 

866 
872 

53 
59 

28 
37 

30 
41 

25 
36 

'  "92'" 

650 

93 

90 

95 

95 

878 

66 

45 

52 

45 

657 

78 

85 

95 

100 

883 

75 

58 

59 

53 

660 

71 

85 

90 

91 

889 

82 

67 

68 

62 

662 

67 

74 

84 

83 

900 

88 

81 

91 

83 

666 

62 

67 

73 

74 

958 

95 

100 

100 

100 

670 

64 

60 

61 

61 

i 

AS  STUDIED   BY  MEANS  OP  THE  RADIOMICROMETER. 


35 


Table  5. — Transmission  oj  Neodymium  Acetate;  Transmission  of  Neodymimn  Sulphate. 


^ 

N.=0.84. 

N.=0.84. 

N.=0.118. 

X 

N.=0.84. 

N.=0.84. 

N.=0.118. 

D.»2.5mm. 

D.=5  mm. 

D.  =  10mm. 

D. =2.5  mm. 

D.=5  mm. 

D.  =  10mm. 

486 

86 

"Ta" 

89   1 

672 

87 

83 

91 

492 

88 

90 

95 

676 

88 

84 

92 

495 

93 

85 

98 

678 

90 

88 

94 

499 

83 

65 

97   i 

681 

94 

93 

95 

501 

73 

50 

86    1 

685 

96 

90 

95 

503 

67 

39 

78 

691 

98 

94 

96 

505 

55 

29 

72 

699 

93 

88 

97 

507 

57 

31 

72 

706 

89 

67 

98 

509 

53 

40 

73 

710 

72 

47 

97 

513 

57 

35 

80 

714 

52 

27 

94 

515 

52 

26 

62 

719 

35 

12 

83 

516 

39 

12 

55 

722 

21 

3 

58 

518 

36 

19 

67 

724 

14 

4 

42 

520 

44 

33 

79 

729 

22 

14 

31 

522 

59 

53 

89 

733 

34 

28 

31 

525 

72 

70 

92 

737 

49 

42 

40 

530 

83 

91 

100 

741 

64 

60 

59 

535 

96 

96 

100 

745 

79 

74 

78 

544 

98 

96 

100 

760 

86 

78 

89 

555 

94 

93 

100 

755 

88 

74 

94 

556 

88 

81 

100 

760 

87 

64 

96 

559 

85 

69 

95 

765 

76 

47 

97 

563 

66 

39 

85 

769 

59 

32 

94 

565 

46 

21 

63 

772 

42 

17 

86 

567 

27 

5 

38 

776 

28 

8 

75 

572 

12 

0 

28 

781 

22 

9 

50 

577 

4 

0 

29 

786 

25 

17 

29 

579 

3 

1 

42 

792 

26 

29 

25 

583 

8 

3 

55 

798 

51 

43 

40 

585 

19 

12 

69 

802 

63 

59 

53 

587 

32 

23 

81 

806 

77 

72 

74 

589 

48 

40 

93 

811 

86 

83 

84 

592 

62 

50 

97 

818 

91 

88 

92 

595 

76 

73 

100 

823 

95 

89 

95 

597 

85 

90 

100 

833 

94 

87 

96 

600 

91 

98 

100 

836 

95 

66 

96 

605 

97 

97 

98 

840 

77 

63 

95 

614 

99 

96 

97 

845 

68 

46 

94 

622 

99 

96 

94 

850 

63 

46 

83 

629 

94 

95 

97 

855 

66 

47 

71 

638 

98 

93 

100 

866 

73 

63 

71 

643 

96 

92 

99 

872 

78 

74 

81 

650 

99 

93 

99 

878 

82 

82 

85 

657 

93 

87 

98 

883 

90 

87 

88 

660 

91 

80 

98 

889 

93 

90 

89 

662 

89 

80 

98 

900 

96 

96 

90 

666 

88 

82 

93 

958 

98 

100 

99 

670 

86 

80 

93 

36       ABSORPTION   SPECTRA   OF   AQUEOUS   SOLUTIONS   OF   CERTAIN   SALTS 

When  we  first  began  to  investigate  any  given  salt  we  made  a  preliminary 
survey  of  its  spectrum,  noting  the  approximate  positions  of  the  absorption 
lines  and  bands.  We  then  made  our  observations  very  close  together  over 
the  regions  in  which  the  preliminary  survey  had  indicated  the  presence  of 
lines  and  bands.  The  number  of  absorption  lines  and  bands,  as  is  well 
known,  is  very  great  in  the  case  of  neodymium  compounds,  and  these  lines 
and  bands  frequently  have  very  sharp  edges.  This  made  the  work  with  this 
substance  very  difficult.  The  proper  width  of  slit  and  position  had  to  be 
chosen  or  a  considerable  error  would  result.  Given  a  slit  width  which  was 
approximately  the  same  as  that  of  an  absorption  line,  a  very  slight  move- 
ment of  the  slit  or  prism  would  change  very  greatly  the  total  amount  of 
energy  falling  on  the  thermal  junction. 

Take  the  neodymium  band  X4275,  which  is  very  intense  but  narrow.  On 
both  sides  of  this  band  there  is  a  region  of  almost  perfect  transparency.  If 
the  slit  width  necessary  to  give  the  desired  deflection  was  greater  than  the 
width  of  this  band,  light  would  pass  through  around  the  edges  of  the  band, 
and  an  error,  which  might  be  of  very  considerable  magnitude,  would  result. 
With  substances  which  did  not  contain  such  fine  lines  and  bands  the  work 
would  be  much  simpler. 

The  entire  spectrum  from  wave-lengths  X4000  to  X20000  was  observed  at 
intervals  of  from  20  a.u.  to  50  a.u.,  except  in  the  regions  where  the  pre- 
liminary survey  indicated  the  absence  of  absorption  lines  and  bands. 

An  examination  of  table  2  will  show  at  X486  a  transparency  [of  72  per 
cent,  which  rapidly  decreases,  reaching  the  first  minimum  at  X505.  There 
the  transparency  amounts  to  only  2  per  cent.  The  transparency  then 
increases  a  little  and  quickly  drops  to  6  per  cent  at  X515.  The  transparency 
then  increases,  becoming  nearly  complete  at  X535.  We  have  here,  then,  a 
double  band  with  greater  absorption  on  the  red  side.  Other  minima  appear 
at  X572,  X730,  X786,  and  X860.  Bands  X730,  X786,  and  X860  do  not  appear 
on  the  photographic  plate,  and  the  last  two  seem  never  to  have  been  detected 
before.     The  above  wave-lengths  are  given  as  in  the  tables. 

The  salts  of  neodymium  were  studied  as  far  as  X20000,  but  beyond  Ifi 
there  seems  to  be  complete  transparency.  The  absorption  of  water  is,  as  is 
well  known,  very  great  in  the  region  X12000  to  X20000. 

DISCUSSION  OF  THE  RESULTS. 

The  results  are  plotted  in  figs.  1  to  11.  The  abscissae  are  percentage  trans- 
parencies, the  ordinates  are  wave-lengths.  These  curves,  since  they  repre- 
sent the  transparencies  of  the  solutions  in  question,  are  called  transmission 
curves. 

Figs.  1,  2,  and  3  represent  the  transparency  of  solutions  of  neodsmiium 
chloride  expressed  in  terms  of  Beer's  law.  If  we  represent  the  concentration 
by  N  and  the  depth  of  layer  by  d, 

Nd=  constant 


AS   STUDIED   BY  MEANS   OF  THE  RADIOMICROMETER.  37 

The  concentration  reprebentcd  in  fig.  1^  is  3.43  normal,  in  fig.  2  it  is  0.857 
normal,  and  in  fig.  3  it  is  0.427  normal.  The  depth  of  layer  represented  by 
fig.  1  is  2.5  mm.,  by  fig.  2  it  is  10  mm.,  and  by  fig.  3  it  is  20  mm.  The  con- 
centration and  depth  of  layer  were  thus  varied  so  as  to  keep  Nd  constant. 

If  the  solvent  plays  no  role  in  the  absorption,  the  three  sets  of  curves  must 
fall  directly  over  one  another,  i.  e.,  be  identical,  since  the  number  of  absorb- 
ing parts  in  the  path  of  the  beam  of  light  is  kept  constant.  A  comparison  of 
the  curves  shows  that,  in  general,  the  more  concentrated  the  solution  the  less 
the  transparency  and  the  broader  the  absorption  bands.  In  the  more  dilute 
solution  the  intensity  of  the  bands  is  greater.  This  comes  out  very  clearly 
in  the  red  and  infra-red  region,  where  there  is  greater  accuracy  of  measure- 
ment. 

Take  the  three  absorption  bands,  X730,  X785,  and  X860.  In  curve  1 
the  minima  of  these  bands  are  approximately  4,  9,  and  33  per  cent,  while 
the  minima  in  curve  2  are  much  less.  In  fig.  2  the  bands  X730  and  X785 
reach  the  abscissa,  which  means  that  there  is  no  transmission.  At  this  dilu- 
tion the  band  X860  has  still  considerable  transparency,  as  will  be  seen  by 
the  fact  that  it  remains  a  considerable  distance  above  the  abscissa.  The 
band  X860  does  not  reach  the  abscissa  even  at  the  dilution  represented  in 
fig.  3. 

All  of  the  bands  manifest  the  above  phenomena,  the  change  in  intensity 
being  greatest  where  the  change  in  dilution  is  greatest,  i.e., from  curve  1  to 
curve  2.  With  increase  in  dilution  the  position  of  the  middle  of  the  band  is 
displaced  toward  the  region  of  greater  wave-length. 

Similar  results  were  obtained  with  neodymium  bromide,  and  these  are 
plotted  in  curves  4,  5,  and  6.  The  concentrations  and  depths  of  layer  were 
varied  so  that  the  product  of  the  two  remained  constant.  The  work  with 
the  bromide  was,  therefore,  done  in  terms  of  Beer's  law.  The  concentra- 
tions used  were  1.66  normal,  0.415  normal,  and  0.208  normal,  the  correspond- 
ing depths  of  the  solution  being  2.5  mm.,  10  mm.,  and  20  mm.  We  find  here 
the  same  general  changes  in  the  intensities  of  the  bands  as  with  the  chloride. 
The  more  dilute  the  solution  the  more  intense  and  the  narrower  the  band. 

This  is  shown  by  comparing  figs.  4,  5,  and  6.  In  fig.  4,  which  represents 
the  most  concentrated  solution  of  the  three,  the  bands  are  the  least  intense. 
In  fig.  5  the  opacity  of  two  of  the  bands  has  become  complete,  shown  by  the 
fact  that  these  touch  the  abscissa. 

Neodymium  nitrate  was  also  studied  and  the  results  are  plotted  in  curves 
7,  8,  and  9.  The  concentrations  used  were  2.95,  0.736,  and  0.368  normal. 
The  depths  of  layer  were  2.5  mm.,  10  mm.,  and  20  mm. 

Band  X570,  curve  7,  appears  to  be  an  exception  to  the  general  relation 
pointed  out  above,  connecting  intensity  and  width  of  band  with  dilution. 
This  was  the  first  band  studied  by  means  of  the  radiomicrometer,  and  com- 
paratively small  deflections  were  observed  in  this  region  of  the  spectrum. 

*  Our  attention  was  drawn  to  the  existence  of  these  bands  in  the  infra-red  by  Pfund, 
who  had  already  mapped  them  radiometrically  for  neodymium  nitrate. 


38      ABSORPTION   SPECTRA   OF  AQUEOUS   SOLUTIONS   OF   CERTAIN  SALTS 

The  remaining  bands  of  neodymium  nitrate,  however,  show  the  same  rela- 
tions that  have  been  pointed  out  for  the  chloride  and  bromide;  with  increas- 
ing dilution  the  intensities  of  the  bands  increase  and  the  centers  seem  to  be 
displaced  somewhat  towards  the  longer  wave-lengths. 

We  then  have  three  salts,  neodymium  chloride,  neodymium  bromide,  and 
neodymium  nitrate,  all  of  which  show  a  marked  increase  in  the  intensity  of 
the  absorption  bands  with  increase  in  dilution,  when  the  product  of  con- 
centration and  depth  of  layer  is  kept  constant,  i.  e.,  when  the  conditions 
demanded  by  Beer's  law  are  fulfilled. 

POSSIBLE  EXPLANATION. 

It  is  well  known  that  a  resonator  vibrates  more  strongly  if  excited  by  the 
vibrations  from  one  single  vibrating  resonator  of  the  same  pitch  than  when 
set  into  vibration  by  a  large  number  of  resonators,  one  of  which  has  the  same 
period  as  its  own,  and  the  others  slightly  different  periods.  In  other  words, 
if  several  vibrators  are  near  one  another,  every  one  exerts  a  certain  influence 
on  its  neighbors.  The  result  is  that  no  one  of  them  has  exactly  the  same 
period  as  the  original  resonator. 

The  presence  of  one  vibrator  seems  to  exercise  a  damping  influence  on  the 
other,  and  causes  it  to  vibrate  with  a  period  slightly  different  from  its  normal 
period.     We  thus  have  less  perfect  resonance. 

The  absorption  of  light  by  solutions  appears  to  be  a  resonance  phenomenon. 
In  a  concentrated  solution  the  vibrators  are  relatively  close  to  one  another 
and  mutually  affect  one  another.  The  result  is  an  imperfect  resonance,  and 
consequently  the  absorption  bands  are  less  intense  in  the  more  concentrated 
solution. 

The  vibrators  are  farther  removed  from  one  another  in  the  more  dilute 
solutions,  and  in  most  cases  are  probably  surrounded  by  large  amounts  of 
water  of  hydration.  The  damping  effect  would  not  be  so  pronounced,  and 
a  resonator  would  have  greater  freedom  to  vibrate  in  its  own  period.  In 
such  cases  we  would  have  a  more  nearly  perfect  resonance,  and  the  resulting 
absorption  bands  would  be  more  intense.  This  tentative  explanation  seems 
to  account  for  the  observed  facts.  Subsequent  work  has  shown  that  a  part 
of  this  effect  can  be  explained  as  due  to  the  fact  that  the  slit  width  was  not 
infinitesimal.  Fig.  10  is  plotted  from  the  results  for  neodymium  sulphate, 
and  fig.  11  from  those  for  neodymium  acetate.  The  concentration  of  the 
sulphate  is  0.118  normal,  and  of  the  acetate  0.84  normal.  The  length  of  the 
solution  of  the  sulphate  is  10  mm.,  and  of  the  acetate  2.5  mm. 

The  absorption  of  the  acetate,  for  a  given  concentration,  is  much  greater 
than  that  of  any  other  neodymium  salt  thus  far  studied.  This  agrees  with 
the  results  obtained  photographically. 

The  absorption  of  water  beyond  Ifx  is  very  great,  as  has  already  been 
stated.  If  we  are  working  with  very  concentrated  solutions  and  use  a 
"water''  vessel  of  the  same  thickness  as  the  "solution"  vessel,  it  is  obvious 
that  the  results  would  not  be  comparable.    Take  the  3.43  normal  solution 


AS   STUDIED   BY  MEANS   OF  THE  RADIOMICROMETER. 


3d 


of  neodymium  chloride;  it  contains,  for  a  given  thickness,  only  about  90  per 
cent  as  much  water  as  the  same  thickness  of  pure  water.  It  is,  then,  obvious 
that  in  the  longer  wave-lengths  a  correction  term  must  be  introduced  for  this 
difference.  This  was  practically  negligible  with  salts  of  neodymium,  since 
these  do  not  seem  to  have  any  bands  in  the  region  where  water  has  appre- 
ciable absorption. 

Salts  of  praseodymium  have  bands  in  the  infra-red,  at  least  as  far  as  2/x. 
In  such  cases  the  above  correction  must  be  introduced.  This  correction 
can  be  introduced  in  either  of  two  ways.  We  can  take  the  specific  gravity 
of  the  solution  and  from  the  concentration  calculate  the  amount  of  water 
present.  We  can  then  use  a  "water"  vessel  of  suitable  thickness.  For 
example,  if  the  very  concentrated  solution  in  question  contains  only  90  per 
cent  of  water,  and  we  use  a  vessel  for  the  solution  which  is  10  mm.  thick,  we 
must  use  a  vessel  for  the  water  which  is  only  9  mm.  thick.  In  this  way  the 
beam  of  light  is  made  to  pass  through  the  same  amount  of  water  both  in  the 
case  of  the  solution  and  of  the  solvent,  and  the  absorption  due  to  water  is, 
therefore,  the  same  in  the  two  cases. 

The  second  method  of  procedure  is  to  allow  the  "water"  vessel  and  the 
"solution"  vessel  to  be  of  the  same  thickness,  and  to  apply  mathematically 
the  proper  correction  to  the  results  obtained. 


40      ABSORPTION   SPECTRA   OF  AQUEOUS   SOLUTIONS   OF   CERTAIN  SALTS 


0.6//  0.7/i 

FiQ.  5. 


AS   STUDIED   BY   MEANS   OF   THE   liADlOMlCliOMETEK. 


41 


0.6  ju.  0.7 /LL 

Fig.  8. 


0.5/x  0.6/z  0.7// 

Fig.  9. 


lOOl- 
80 
60 
40 
20 


01 

0.4// 


0.5// 


0.6/i 


0.7/W 

Fig.  10. 


0.8// 


0.9;/ 


1.0// 


CHAPTER  V. 

THE  ABSORPTION  OF  LIGHT  BY  WATER  CHANGED  IN  THE 
PRESENCE  OF  STRONGLY  HYDRATED  SALTS.  AS  SHOWN  BY 
THE  RADIOMICROMETER— NEW  EVIDENCE  FOR  THE  SOLVATE 
THEORY  OF  SOLUTION. 

The  use  of  the  radiomicrometer  in  studying  the  absorption  spectra  of  cer- 
tain substances  has  already  been  discussed  by  Jones  and  Guy.^  The  radio- 
micrometer  was  used  for  studying  the  absorption  spectra  of  solutions 
rather  than  the  grating  spectrograph  and  the  photographic  plate,  because  it 
enabled  us  to  measure  not  only  the  positions  of  the  different  lines  and  bands, 
but  also  to  study  quantitatively  their  intensity;  and  also  because  it  made 
possible  the  study  of  the  absorption  spectra  of  solutions  over  a  much  greater 
range  of  wave-lengths  than  the  photographic  method. 

In  building  a  radiomicrometer  adapted  to  this  work — that  is,  with  suffi- 
cient sensibihty  and  with  a  short  period — one  of  the  greatest  difficulties,  as 
already  mentioned,  was  to  obtain  copper  wire  free  from  iron.  This  was  a 
necessity,  since  the  presence  of  an  appreciable  quantity  of  iron  in  the  copper 
gave  rise  to  a  "magnetic  control"  which  rendered  the  instrument  unstable 
and  the  zero-point  inconstant.  This  difficulty  was  for  the  most  part  over- 
come, due  to  the  kindness  of  Messrs.  Leeds  and  Northrup  of  Philadelphia 
and  of  R,.  W.  Paul  of  London.  Both  of  these  houses  furnished  us  with 
copper  wire  so  free  from  iron  that  the  "magnetic  control"  could  easily  be 
regulated.  By  means  of  this  wire  and  the  thermo-electric  junction  already 
described,  a  most  sensitive  radiomicrometer  was  built,  which  at  the  same 
time  had  a  very  short  period,  and  with  this  instrument  work  was  done  with 
salts  of  neodymium  and  praseodymimn,  the  results  of  which  were  recorded 
in  the  Physikalische  Zeitschrift.^ 

ABSORPTION  OF  FREE  AND  COMBINED  WATER. 

At  the  beginning  of  the  academic  year  1912-13  the  absorption  spectra 
of  solutions  of  a  large  number  of  salts  of  different  metals  were  mapped  out 
and  compared  with  the  absorption  of  water,  using  the  same  depths  of  water 
as  the  water  in  the  various  solutions.  The  depth  of  water  in  the  solution 
was  determined  from  the  concentration  of  the  solution  and  from  its  specific 
gravity.  It  was  soon  found  that  the  absorption  of  the  solution  was  less^  and 
in  many  cases  very  much  less  than  that  of  the  layer  of  water  having  a  depth 
equal  to  the  depth  of  the  water  in  the  solution. 

The  above  result  is  directly  at  variance  with  everything  that  was  known 
at  the  time.  The  dissolved  substance  could  not  have  less  than  no  absorp- 
tion of  light,  the  assumption  having  been  made  up  to  this  time  that  in  an 

1  Phys.  Zeit.,  13,  649  (1912).  *  Ibid.,  13,  651  (1912). 

43 


44  ABSORPTION   OF  LIGHT  BY  WATER   CHANGED 

aqueous  solution  the  water  present  absorbs  just  as  much  as  pure,  uncom- 
bined  water. 

It  became  at  once  obvious  that  we  could  not  measure  the  absorption 
spectrum  of  a  solution,  subtract  from  it  the  absorption  due  to  water,  and 
conclude  that  the  remainder  was  the  absorption  due  to  the  dissolved  sub- 
stance ;  since  the  water  in  the  solution  has  very  different  absorption  from  an 
equal  amount  of  pure,  uncombined  water. 

We  then  carried  out  a  number  of  experiments  in  cells  whose  depths  could 
be  easily  and  accurately  adjusted,  with  different  substances,  in  the  following 
manner:  The  absorption  spectra  of  a  number  of  different  substances  were 
first  measured,  then  the  absorption  spectra  of  water  having  the  same  depths 
of  layer  as  the  water  in  the  solutions.  For  certain  substances  the  pure 
water  was  more  opaque  than  the  solutions,  and  for  other  substances  the 
water  was  more  transparent.  The  percentage  transmission — that  is,  the 
deflection  of  the  radiomicrometer  for  the  solution,  divided  by  the  deflection 
for  water — for  the  first-named  substances  amounted  to  more  than  100  per 
cent.  Pure  water  had  a  different  absorption  from  an  equal  depth  of  water 
in  the  solution,  and  since  this  difference  varied  from  one  dissolved  substance 
to  another,  it  is  obvious  that  this  method  was  not  the  one  to  be  followed. 
It  would  be  very  difficult,  not  to  say  impossible,  to  interpret  the  results 
obtained  by  dividing  the  radiomicrometer  deflections  for  the  solution  by 
those  for  pure  water.  We  should  simply  be  obtaining  the  transmission  of 
the  solution  in  terms  of  pure  water,  which  was  not  what  was  desired. 

What  we  want  to  know  is  the  actual  absorption  or  transmission  of  the  solu- 
tion, and  then  that  of  pure  water  having  a  depth  of  layer  that  was  just  equal 
to  that  of  the  water  in  the  solution.  These  two  sets  of  results  could  then  be 
compared  with  one  another. 

HYDRATED  AND  NONHYDRATED  SUBSTANCES. 

In  this  earlier  work  we  had,  however,  noted  that  solutions  of  those  sub- 
stances which  are  largely  hydrated  are  more  transparent  than  pure  water 
having  the  depths  of  the  water  in  the  solutions  in  question.  Solutions  of 
nonhydrated  substances,  or  of  only  shghtly  hydrated  substances,  provided 
the  substances  themselves  do  not  absorb  light,  are  not  more  transparent 
than  pure  water  having  the  same  depths  as  the  water  in  the  solution.  It 
would  seem  from  this  observation  that  water  combined  with  the  dissolved 
substance  had  less  absorption  of  light  than  pure,  uncombined  water.  To 
test  this  quantitatively  the  following  procedure  was  adopted. 

METHOD  OF  PROCEDURE. 

A  solution  of  the  substance  in  question  was  prepared  of  known  concen- 
tration and  its  specific  gravity  determined.  This  solution  was  placed  in  one 
cell  set  to  a  depth  of  say  21  mm.  Some  of  the  same  solution  was  then 
placed  in  another  cell  set  to  a  depth  of  say  1  mm.  Light  of  a  known  wave- 
length was  then  passed  through  the  one  solution  and  the  deflection  noted. 
Light  of  this  same  wave-length  was  then  passed  at  once  through  the  other 


IN   THE  PRESENCE   OF   STRONGLY  HYDRATED   SALTS.  45 

solution  and  the  deflection  in  this  case  also  noted.  The  deflection  produced 
when  the  deeper  solution  was  in  the  path  of  the  beam  of  light  was  then 
divided  by  the  deflection  produced  by  the  shallower  solution,  and  this  gave 
the  absolute  transmission  of  the  solution  of  the  substance  in  question  of 
known  concentration,  having  a  depth  of  layer  of  20  mm. 

This  process  was  repeated  for  the  different  parts  of  the  spectrum,  chang- 
ing the  wave-length  of  light  from  reading  to  reading  by  only  a  small  amount. 
The  object  of  using  the  two  depths  of  the  same  solution,  and  then  dividing 
the  deflection  produced  by  the  deeper  layer  by  that  obtained  when  the  more 
shallow  layer  was  in  the  path  of  the  beam  of  light,  was  to  eliminate  any 
effect  of  reflection  from  the  glass  ends  closing  the  cells  containing  the  solu- 
tions, and  also  to  eliminate  any  changes  in  the  total  amounts  of  energy  sent 
through  the  solution,  due  to  slight  changes  in  the  intensity  of  the  Nernst 
glower.  From  the  specific  gravity  of  the  solution  and  its  known  concentra- 
tion, the  amount  of  water  in  a  layer  of  the  solution,  say  21  mm.  in  depth, 
could  easily  be  calculated.  Similarly,  the  amount  of  water  in  a  layer  of  the 
solution  which  was  1  mm.  deep,  could  also  be  calculated.  Water  was  then 
introduced  into  two  cells,  and  the  cells  so  adjusted  that  the  difference  in 
depths  was  exactly  equal  to  the  depth  of  the  water  in  the  layer  of  the  solu- 
tion, which  was  20  mm.  deep. 

The  deflection  for  the  water  in  the  deeper  cell  was  then  read  for  any  given 
wave-length  of  light,  and  then,  at  once,  the  deflection  when  the  light  was 
passed  through  the  more  shallow  layer  of  water.  The  deflection  for  the 
deeper  layer  was  divided  by  the  deflection  for  the  shallower  layer.  The 
result  was  the  absolute  transmission  for  water  with  a  depth  of  layer  just 
equal  to  the  depth  of  water  in  the  solution  in  question. 

RESULTS.  -~^ 

The  above  results  for  the  solution  are  plotted  as  one  curve  and  those  for 
water  having  the  same  depth  as  the  water  in  the  solution  as  another  curve, 
wave-lengths  being  abscissae  and  transmission  ordinates.  A  comparison 
of  the  two  curves  shows  at  once  whether  water  in  the  free,  uncombined  con- 
dition or  the  same  depth  of  water  in  the  solution  in  question  is  the  more 
transparent. 

The  data  obtained  by  dividing  the  deflections  produced  by  the  deeper 
solutions  by  those  for  the  shallower,  and,  similarly,  by  those  for  water,  are 
also  given  in  tables  6  to  10.  These  are  the  data  from  which  the  accom- 
panying curves  were  plotted. 

The  substances  studied  were  chosen  from  the  standpoint  of  their  power  to 
solvate  or  to  combine  with  the  solvent  in  which  they  were  dissolved.  In  all 
of  the  work  recorded  in  this  paper  the  solvent  used  was  water.  We  were 
practically  limited,  in  this  phase  of  the  work,  to  those  substances  which 
themselves  have  little  or  no  power  to  absorb  light,  and  which  are  both  color- 
less in  the  visible  part  of  the  spectrum,  and  have  little  or  no  absorption  in 
the  regions  in  which  the  absorption  bands  of  water  occur. 


46 


ABSORPTION   OF  LIGHT  BY   WATER   CHANGED 


We  selected  for  these  substances  with  Uttle  or  no  hydrating  power,  salts 
of  potassium  and  ammonium.  The  potassium  salts  studied  were  the  chlo- 
ride and  nitrate.  Ammonium  chloride  and  nitrate  were  also  investigated. 
For  the  salts  with  large  hydrating  power,  calcium  chloride,  magnesium 
chloride,  and  aluminium  sulphate  were  used.  These  salts  were  shown,  from 
our  earlier  work,  to  be  among  the  most  strongly  hydrated  substances  with 
which  we  are  familiar.     Two  depths  of  layer  of  each  solution  of  every 

Table  6. 


X 

KCl,  4N. 
I/h 

H2O. 

1 
1 

NH4C1,4N. 

Ilh 

H2O. 

NH4NO3, 
i  3.12  N. 

i   Ilh 

H2O. 

711 

97 

97 

92 

98   1 

95 

98 

724 

96 

95 

91 

96 

97 

98 

741 

95 

95 

90 

92 

96 

96 

760 

93 

95 

86 

92 

91 

95 

776 

92 

95 

85 

88 

92 

94 

798 

94 

95 

88 

96 

91 

95 

818 

92 

95 

87 

96 

92 

96 

836 

94 

93 

87 

96 

91 

94 

855 

91 

90 

86 

93 

89 

92 

878 

92 

90 

86 

91 

90 

93 

900 

90 

89 

84 

87 

89 

88 

922 

87 

86 

82 

86 

86 

90 

947 

82 

84 

79 

82 

82 

83 

958 

77 

78 

73 

72 

73 

79 

964 

75 

73 

69 

70 

70 

71 

969 

65 

65 

64 

63 

66 

67 

974 

58 

56 

67 

68 

68 

69 

979 

51 

50 

62 

62 

60 

64 

982 

47 

46 

46 

46 

44 

46 

985 

41 

45 

43 

43 

40 

44 

991 

39 

43 

41 

43 

39 

43 

1,007 

39 

43 

41 

42 

39 

44 

1,013 

40 

42 

39 

44 

!    41 

44 

1,019 

42 

46 

40 

46 

42 

44 

1,025 

41 

42 

44 

49 

45 

48 

1,032 

49 

49 

44 

49 

48 

49 

1,037 

53 

62 

66 

66 

62 

53 

1,042 

56 

66 

63 

68 

66 

55 

1,046 

59 

60 

67 

67 

67 

60 

1,059 

63 

62 

68 

66 

60 

65 

1,065 

68 

67 

62 

67 

64 

65 

1,072 

71 

68 

64 

68 

66 

69 

1,078 

74 

72 

67 

67 

66 

72 

1,085 

75 

73 

67 

66 

68 

74 

1,100 

77 

75 

68 

72 

69 

78 

1,113 

76 

76 

69 

72 

69 

78 

1,138 

75 

72 

68 

70 

68 

72 

1,148 

70 

69 

64 

66 

64 

71 

1,158 

64 

63 

62 

64 

59 

64 

1,165 

58 

59 

58 

68 

66 

60 

1,172 

52 

51 

50 

60 

63 

52 

1,179 

1    42 

40 

40 

40 

38 

41 

1,186 

'    29 

28 

29 

26 

29 

30 

1,193 

18 

19 

19 

19 

18 

19 

1,200 

13 

16 

14 

13 

12 

17 

1,206 

i    10 

12 

12 

13 

9 

13 

1,213 

10 

11 

10 

12 

9 

13 

1,220 

10 

11 

10 

11 

9 

12 

1,227 

10 

11 

10 

11 

9 

12 

IN  THE  PRESENCE   OF   STRONGLY  HYDRATED   SALTS. 


47 


substance  investigated  were  employed,  in  order  to  bring  out  the  two  most 
important  water-bands  in  the  region  of  the  spectrum  used.  This  could 
not  be  done  by  studying  only  one  depth  of  solution,  since  the  depth  which 
was  necessary  and  sufficient  to  bring  out  clearly  one  of  these  water-bands 
would  not  bring  the  other  out  in  the  way  desired.  By  using  the  two  depths 
of  solution,  and  studying  them  in  the  manner  above  described — that  is, 
by  the  differential  method —  we  were  able  to  study  both  of  the  water-bands 
as  produced,  on  the  one  hand  by  the  pure  solvent,  and  on  the  other  by  the 
solution. 

In  tables  6  to  10,  under  X,  are  given  the  wave-lengths  of  light  that  were 
passed  through  the  solution;  and  under  7/7o  the  percentage  of  transmission, 
on  the  one  hand,  of  the  solution;  and  on  the  other,  of  water  having  a  depth 
exactly  equal  to  that  of  the  water  in  the  solution. 

Table  7. 


X 

KCl,  4  N. 

Ilh 

HjO. 

NH4C1,4N. 

Ilh 

H,0. 

NH4NO,, 

3.12  N. 

Ilh 

H,0. 

1,085 

85 

86 

79 

87 

81 

88 

1,100 

87 

88   ! 

80 

92 

81 

93 

1,113 

86 

87   1 

79 

86 

84 

86 

1,138 

81 

85   1 

79 

84 

81 

84 

1,148 

79 

82 

77 

84 

78 

84 

1,158 

80 

79 

74 

81 

76 

81 

1,165 

76 

77 

71 

77 

72 

77 

1,172 

72 

71 

66 

70 

67 

70 

1,179 

64 

62 

69 

62 

61 

62 

1,186 

51 

51 

52 

50 

50 

50 

1,193 

41 

43 

42 

44 

40 

44 

1,200 

35 

38 

37 

40 

34 

40 

1,206 

37 

36 

35 

37 

32 

37 

1,213 

30 

34 

30 

36 

32 

36 

1,220 

30 

35 

29 

35 

34 

35 

1,227 

30 

34 

30 

35 

32 

35 

1,233 

30 

33 

28 

35 

31 

35 

1,241 

30 

34 

29 

34 

31 

34 

1,248 

31 

34 

28 

33 

1    31 

33 

1,250 

33 

34 

30 

34 

32 

34 

1,255 

34 

35 

30 

36 

32 

36 

1,268 

34 

35 

31 

37 

32 

37 

1,270 

37 

37 

38 

38 

38 

38 

1,285 

38 

38 

33 

38 

33 

38 

1,295 

39 

38 

32 

38 

33 

38 

1,300 

41 

38 

32 

39 

34 

39 

1,308 

42 

39 

32 

41 

35 

41 

1,316 

41 

39 

32 

40 

34 

40 

1,323 

42 

37 

32 

38 

34 

38 

1,330 

40 

37 

32 

37 

33 

37 

1,338 

40 

35 

30 

35 

!    31 

35 

1,346 

36 

33 

28 

36 

!    30 

36 

1,352 

34 

29 

26 

29 

j    27 

29 

1,358 

29 

26 

23 

27 

I    25 

27 

1,365 

25 

22 

21 

23 

21 

23 

1,372 

20 

18 

17 

20 

17 

20 

1,387 

13 

12 

12 

11 

10 

11 

1,404 

7 

7 

7 

8 

7 

8 

1,418 

3 

4 

4 

3 

3 

3 

1,430 

2 

2 

3 

2 

2 

2 

1,445 

0 

0 

0 

1 

1 

1 

48 


ABSORPTION   OF  LIGHT  BY  WATER   CHANGED 


In  table  6  the  depth  of  layer  of  all  the  solutions  was  the  difference  between 
21  and  1,  i.  e.,  20  mm.  The  depth  of  water  was  in  every  case  the  same  as 
that  of  the  water  in  the  solution  in  question. 

The  depth  of  layer  of  the  solutions  given  in  table  7  was  the  difference 

between  11  mm.  and  1  mm.,  i.  g.,  10  mm.,  and  was  only  half  of  that  in 

table  6.     The  object  of  this  was  to  bring  out  more  prominently  the  second 

water-band.     The  depth  of  water  used  was  in  every  case  the  same  as  that 

of  the  water  in  the  solution. 

Table  8. 


CaCl., 

MgCl2, 

Al2(S04)3, 

X 

5.38  N. 

I/Io 

H2O. 

4.96  N. 

H2O. 

1.012  N. 
Ilh 

H2O. 

710 

94 

98 

95 

98 

95 

93 

724 

92 

98 

98 

98 

95 

95 

741 

90 

95 

95 

98 

94 

93 

760 

87 

94 

94 

98 

92 

93 

776 

88 

93 

92 

97 

93 

95 

798 

91 

96 

93 

94 

92 

90 

818 

93 

99 

90 

90 

93 

92 

836 

92 

97 

1    92 

95 

92 

92 

855 

90 

93 

!    91 

90 

1    90 

91 

878 

90 

90 

1    91 

93 

91 

90 

900 

89 

92 

!    88 

92 

89 

90 

922 

86 

91 

88 

91 

85 

86 

947 

87 

84 

:    84 

86 

82 

81 

958 

78 

79 

1    76 

78 

76 

73 

964 

75 

73 

82 

76 

72 

66 

969 

70 

68 

75 

69 

68 

61 

974 

65 

62 

68 

65 

64 

55 

979 

59 

53 

61 

56 

58 

48 

982 

51 

49 

48 

51 

53 

42 

985 

48 

49 

54 

45 

51 

40 

991 

44 

46 

1    48 

49 

47 

39 

1,007 

42 

46 

1    46 

48 

46 

38 

1,013 

42 

46 

1    45 

50 

46 

39 

1,019 

43 

49 

1    44 

51 

44 

40 

1,025 

47 

50 

1    46 

44 

46 

43 

1,032 

52 

53 

51 

54 

46 

45 

1,037 

55 

55 

52 

56 

52 

50 

1,042 

58 

59 

56 

58 

53 

53 

1,046 

62 

62 

59 

65 

55 

55 

1,059 

66 

65 

63 

67 

55 

68 

1,065 

71 

70 

69 

70 

62 

63 

1,072 

74 

72 

71 

75 

60 

65 

1,078 

75 

74 

71 

76 

64 

69 

1,085 

78 

76 

76 

79 

65 

70 

1,100 

80 

77 

i    78 

79 

67 

72 

1,113 

79 

78 

i    80 

81 

67 

74 

1,138 

77 

75 

1    77 

78 

64 

67 

1,148 

74 

71 

1    75 

77 

60 

65 

1,158 

69 

65 

1    73 

73 

57 

55 

1,165 

66 

62 

65 

65 

55 

53 

1,172 

61 

52 

61 

58 

50 

43 

1,179 

54 

41 

52 

44 

45 

34 

1,186 

42 

30 

43 

32 

34 

22 

1.193 

32 

21 

32 

24 

25 

15 

1,200 

22 

17 

23 

18 

20 

12 

1,206 

16 

16 

18 

17 

16 

10 

1,213 

13 

15 

16 

18 

14 

9 

1,220 

12 

13 

14 

15 

11 

10 

1,227 

12 

13 

14 

16 

12 

8 

IN   THE   PRESENCE   OF   STRONGLY   HYDRATED   SALTS. 


40 


The  depth  of  layer  of  the  solutions  given  in  table  8  was  the  difference 
between  21  mm.  and  1  mm.,  i.  e.,  20  mm.  The  depth  of  water  used  in  every 
case  was  the  same  as  that  of  the  water  in  the  solution. 

In  table  9  the  depth  of  layer  used  was  the  difference  between  11  and  1  mm., 
I.  6.,  10  mm.  The  object  of  using  the  smaller  depth  of  the  solution  was  to 
bring  out  more  clearlj^  in  the  case  of  hydrated  salts  the  second  water-band. 

When  salts  which  are  strongly  hydrated  in  aqueous  solution  are  not  very 
concentrated,  the  difference  between  the  transparency  of  the  salt  solution 
and  that  of  water  of  the  same  depth  of  layer  as  the  water  in  the  solution  is 
not  so  pronounced.  This  is  what  would  be  expected,  since  the  total  amount 
of  water  combined  with  the  dissolved  salt  increases  with  the  concentration  of 
the  solution.     The  data  given  in  table  10  bring  out  this  fact. 

Table  9. 


CaCl2, 

MgCU, 

Al2(S04)3, 

X 

5.38  N. 
I/Io 

H2O. 

4.96  N. 
I/h 

H2O. 

1.02  N. 
I/Io 

H2O. 

1,085 

84 

84 

82 

84 

79 

82 

1,100 

84 

84 

83 

84 

78 

81 

1,113 

84 

86 

83 

84 

78 

84 

1,138 

86 

85 

82 

83 

77 

83 

1,148 

82 

83 

79 

80 

75 

80 

1,158 

80 

79 

77 

77 

73 

77 

1,165 

78 

76 

77 

75 

70 

73 

1,172 

76 

72 

74 

70 

66 

69 

1,179 

72 

65 

71 

64 

63 

68 

1,186 

63 

55 

62 

52 

55 

48 

1,193 

64 

45 

56 

46 

49 

39 

1,200 

45 

40 

48 

42 

43 

34 

1,206 

38 

39 

42 

40 

38 

30 

1,213 

35 

39 

39 

38 

36 

29 

1,220 

33 

36 

37 

88 

34 

28 

1,227 

32 

36 

36 

38 

32 

28 

1,233 

31 

35 

34 

37 

32 

28 

1,241 

32 

35 

34 

34 

31 

28 

1,248 

32 

35 

34 

37 

1    31 

28 

1,250 

33 

35 

34 

38 

1    31 

28 

1,255 

33 

37 

34 

38 

31 

28 

1,268 

35 

38 

33 

38 

30 

29 

1,270 

37 

39 

34 

39 

30 

30 

1,285 

38 

40 

35 

40 

30 

31 

1,295 

40 

40 

35 

40 

1    30 

32 

1,300 

42 

41 

36 

41 

1    30 

32 

1,308 

42 

41 

37 

42 

30 

33 

1,316 

45 

41 

39 

42 

30 

33 

1,323 

47 

40 

39 

41 

28 

33 

1,330 

46 

39 

40 

40 

27 

32 

1,338 

45 

37 

38 

38 

27 

30 

1,346 

42 

35 

38 

36 

24 

27 

1,352 

40 

32 

34 

33 

22 

24 

1,358 

37 

29 

33 

30 

20 

21 

1,365 

33 

25 

29 

26 

18 

19 

1,372 

29 

21 

25 

22 

15 

15 

1,387 

19 

13 

18 

15 

10 

9 

1,404 

12 

10 

12 

11 

7 

5 

1,418 

7 

6 

7 

9 

4 

3 

1,430 

3 

3 

4 

3 

2 

2 

1,445 

2 

1 

1 

1 

1 

1 

50 


ABSORPTION   OF  LIGHT  BY  WATER   CHANGED 


The  depth  of  layer  of  the  different  solutions  for  which  the  results  are 
recorded  in  table  10  was  the  difference  between  21  and  1  mm.,  i.  e.,  20  mm. 
The  results  are,  therefore,  comparable  with  those  recorded  in  table  8,  the 
difference  being  a  difference  in  the  concentrations  of  the  solutions  used. 
The  difference  between  the  transmission  of  the  solution  and  that  of  water  at 
the  same  depth  as  the  water  in  the  solution  is  very  much  less  for  the  more 


Table  10. 


CaCla 

MgCl, 

AIjCSO*), 

X 

2.69  N. 
I/h 

HaO. 

2.48  N. 
I/h 

H2O. 

0.508  N. 
I/h 

H2O. 

710 

96 

94 

95 

95 

97 

96 

724 

95 

96 

93 

96 

98 

96 

741 

95 

95 

90 

95 

95 

93 

760 

94 

96 

92 

95 

95 

95 

776 

93 

97 

93 

95 

95 

95 

798 

90 

98 

91 

95 

96 

96 

818 

93 

97 

91 

93 

95 

96 

836 

91 

96 

89 

93 

93 

95 

855 

91 

92 

88 

92 

92 

92 

878 

90 

92 

84 

90 

90 

91 

900 

88 

90 

84 

88 

89 

86 

922 

89 

92 

81 

86 

82 

85 

947 

82 

86 

78 

83 

78 

80 

958 

75 

79 

72 

76 

73 

75 

964 

70 

74 

1    70 

73 

68 

69 

969 

65 

69 

62 

64 

62 

62 

974 

58 

61 

58 

58 

57 

54 

979 

50 

52 

50 

51 

50 

46 

982 

44 

47 

46 

46 

46 

42 

985 

40 

43 

42 

43 

43 

40 

991 

39 

41 

'    40 

41 

41 

39 

1,007 

38 

40 

1    41 

42 

40 

40 

1,013 

39 

42 

!    40 

44 

40 

40 

1,019 

40 

43 

40 

44 

41 

41 

1,025 

43 

45 

44 

41 

43 

43 

1,032 

45 

47 

i    47 

44 

45 

46 

1,037 

48 

50 

;    50 

46 

47 

48 

1,042 

51 

52 

1    52 

48 

49 

49 

1,046 

66 

56 

!    56 

54 

53 

54 

1,059 

61 

59 

1    58 

55 

60 

59 

1,065 

65 

64 

!    64 

62 

59 

62 

1,072 

69 

67 

1    67 

64 

63 

65 

1,078 

70 

69 

69 

67 

65 

69 

1,085 

72 

72 

72 

68 

68 

72 

1,100 

73 

73 

73 

71 

69 

73 

1,113 

72 

74 

74 

72 

68 

74 

1,138 

72 

74 

74 

70 

67 

72 

1,148 

66 

69 

69 

67 

64 

67 

1,158 

67 

62 

62 

60 

58 

62 

1,165 

57 

58 

58 

58 

54 

54 

1,172 

52 

51 

53 

52 

47 

46 

1,179 

46 

39 

42 

42 

39 

35 

1,186 

30 

27 

31 

27 

28 

25 

1,193 

20 

19 

21 

20 

20 

16 

1,200 

13 

14 

15 

15 

14 

12 

1,206 

1    12 

11 

12 

13 

12 

10 

1,203 

1    11 

11 

12 

12 

11 

10 

],220 

11 

10 

11 

11 

11 

10 

1,227 

10 

9 

10 

10 

10 

9 

IN  THE  PRESENCE  OF  STRONGLY  HYDRATED  SALTS. 


51 


dilute  than  for  the  more  concentrated  solutions;  this  is  what  would  be 
expected  in  terms  of  the  solvate  theory  applied  to  the  phenomenon  in 
question. 

Considerable  work  was  done  in  comparing  directly  the  transmission  of  a 
solution  and  that  of  water  having  the  same  depth  as  the  water  in  the  solu- 
tion in  question.  The  deflection  of  the  radiomicrometer  as  given  by  the 
solution  is  in  the  column  marked" Deflection  of  solution," and  the  deflection 
as  given  by  water  having  the  same  depth  as  water  in  the  solution  is  given 
in  column  ''Deflection  of  water." 


Table  11. 


X 

Deflection 
of  solution 

of 
Ab  (SOi),. 

Deflection 
of  water. 

Deflection 

of  solution 

of  KCl. 

j 
Deflection 
of  water. 

\ 

Deflection 
of  solution 

of 
Ab  (SOi)*. 

Deflection 
of  water. 

Deflection 

of  solution 

of  KCI. 

Deflection 
of  water. 

710 

50 

61 

53 

53 

1,037 

91 

84 

112 

108 

724 

58 

58 

56 

56 

1,042 

92 

92 

119 

116 

741 

62 

63 

67 

68 

1,046 

99 

99 

125 

120 

760 

72 

72 

77 

77 

1,059 

105 

105 

141 

136 

776 

75 

76 

88 

90 

1,065 

109 

112 

150 

145 

798 

83 

83 

98 

99 

1,072 

114 

119 

159 

153 

818 

82 

82 

108 

109 

1,078 

118 

125 

164 

158 

836 

93 

94 

116 

116 

1,085 

122 

132 

168 

164 

855 

97 

97 

124 

124 

1,100 

128 

140 

176 

172 

878 

105 

105 

129 

130 

1,113 

129 

142 

178 

175 

900 

105 

105 

140 

138 

1,138 

127 

142 

174 

170 

922 

112 

112 

140 

140 

1,148 

123 

131 

164 

162 

947 

113 

113 

142 

142 

1,158 

112 

118 

161 

159 

958 

109 

106 

136 

136 

1,165 

108 

111 

157 

154 

964 

107 

100 

129 

125 

1,172 

99 

94 

132 

126 

969 

104 

93 

118 

116 

1,179 

87 

74 

107 

100 

974 

98 

83 

108 

106 

1,186 

68 

49 

73 

66 

979 

93 

73 

92 

92 

1,193 

54 

35 

50 

48 

982 

82 

66 

83 

83 

1,200 

42 

26 

34 

36 

985 

80 

64 

78 

80 

1,206 

35 

23 

29 

32 

991 

78 

62 

78 

80 

1,213 

30 

21 

25 

30 

1,007 

78 

65 

78 

81 

1,220 

28 

20 

24 

29 

1,013 

74 

65 

81 

85 

1,227 

26 

19 

24 

28 

1,019 

77 

68 

84 

88 

1,241 

24 

19 

23 

26 

1,025 

80 

75 

96 

96 

1,255 

23 

18 

26 

27 

1,032 

84 

77 

100 

101 

The  results  obtained  for  aluminium  sulphate  having  a  concentration 
1.02  N,  and  for  potassium  chloride  4  N  are  given  in  table  11.  The  depth 
of  solution  used  was  20  mm.,  and  the  depth  of  water  that  of  the  water  in  the 
solutions  in  question. 

Duplicate  measurements  were  made  with  the  radiomicrometer  for  nearly 
every  solution  of  all  the  substances  worked  with  at  the  various  wave-lengths 
studied.  It  was  found  that  readings  for  the  different  solutions  of  the  same 
substance  having  the  same  concentration  were,  for  a  given  wave-length, 
different  from  one  another  to  the  extent  of  somewhat  less  than  2  per  cent. 
From  this  it  seems  fair  to  assume  that  the  error  in  our  work  was  not  greater 
than  2  per  cent. 


62  ABSORPTION   OF   LIGHT  BY  WATER   CHANGED 


DISCUSSION  OF  THE  RESULTS. 

An  examination  of  the  tables  of  data  for  potassium  chloride,  ammonium 
chloride,  and  ammonium  nitrate — that  is,  for  those  substances  which,  in 
aqueous  solutions,  combined  with  very  little  water,  as  was  demonstrated  by 
the  freezing-point  method,  shows  that  for  all  wave-lengths  studied  the  solu- 
tion, and  water  of  the  same  depth  as  the  water  in  the  solution,  have  prac- 
tically the  same  transmission.  The  dissolved  substance  does  not  combine 
with  the  solvent  water,  and  the  water  in  the  solution  has  almost  exactly  the 
same  effect  upon  light  as  so  much  pure  water  would  have.  This  is  exactly 
what  would  be  expected  from  our  knowledge  of  the  absorption  of  light  by 
dissolved  substances  and  by  the  solvent.  When  we  began  this  work  we 
supposed,  as  others  had  done,  that  the  water  in  the  solution,  whether  it  was 
combined  with  the  dissolved  substance  or  not,  would  have  the  same  power 
to  absorb  light  as  so  much  pure  solvent  water.  We  shall  now  see  that  such 
is  not  the  case. 

The  results  for  the  above-named  substances  were  not  plotted  in  the  form 
of  curves,  since  the  curve  for  water  and  for  the  solution  would  practically 
coincide  with  one  another,  the  dissolved  substance  having  very  little  absorp- 
tion over  the  region  of  wave-lengths  studied  in  this  investigation. 

When  we  turn  to  the  data  in  tables  8  and  9  very  different  relations  mani- 
fest themselves.  These  are  the  data  for  calcium  chloride,  magnesium  chlo- 
ride, and  aluminium  sulphate,  that  is,  for  salts  which,  in  aqueous  solution, 
are  strongly  hydrated,  as  was  shown  by  the  earlier  work  in  this  laboratory.^ 
The  solution  in  these  cases  is  often  more  transparent  than  the  same  amount 
of  water  that  is  contained  in  the  solution. 

That  these  relations  may  appear  the  more  clearly,  the  results  obtained  for 
the  above-named  salts  are  plotted  as  curves  in  figs.  12  to  17.  Fig.  12  is  the 
curve  for  calcium  chloride  having  a  depth  of  20  mm.  This  was  obtained  by 
dividing  the  deflection  produced  by  21  mm.  of  the  solution  by  that  pro- 
duced by  1  mm.  of  the  solution.  On  the  same  sheet  we  have  the  curve  for 
water  having  a  depth  equal  to  that  of  the  water  in  the  calcium  chloride. 
This  curve  for  water  was  also  obtained  by  the  ''differential"  method,  i.  e.,  by 
dividing  the  deflections  produced  by  the  deeper  solution  by  those  obtained 
with  the  more  shallow  solution,  the  difference  in  the  depths  of  water  in  the 
two  cases  being  just  equal  to  the  depth  of  water  in  20  mm.  of  the  solution  in 
question.  Fig.  13  is  the  curve  for  calcium  chloride  with  a  depth  of  layer  of 
10  mm.  (11  —  1).  The  data  from  which  the  curve  was  plotted  are  contained 
in  table  9.  The  smaller  depth  of  solution  was  used,  so  that  the  water-band 
between  1.2/z  and  1.3/i  would  come  out  more  distinctly.  The  results  for  this 
solution,  like  those  for  all  the  others,  are  compared  with  the  absorption  of 
a  depth  of  water  equal  to  that  of  the  water  in  the  solution.  The  absorption 
of  the  water,  in  this  as  in  all  other  cases,  was  obtained  by  the  "differential' • 
method. 

1  Cam.  Inst.  Wash.  Pub.  60. 


IN   THE   PRESENCE   OP  STRONGLY   HYDRATED   SALTS.  63 

Fig.  14  is  the  curve  for  magnesium  chloride  having  a  depth  of  21  —  1  = 
20  mm.,  and  the  corresponding  water-curve.  The  data  from  which  these 
curves  are  plotted  are  given  in  table  8. 

Fig.  15  is  the  curve  for  magnesium  chloride  having  a  depth  of  1  cm.,  also 
obtained  by  the  ' 'differential"  method.    These  data  are  taken  from  table  9. 

Fig.  16  is  the  curve  for  aluminium  sulphate  having  a  depth  of  21  — 1  = 
20  mm.,  and  the  corresponding  absorption  curve  for  water. 

Fig.  17  is  the  curve  for  aluminium  sulphate  having  a  depth  of  11  —  1  = 
10  mm.,  and  the  corresponding  water-curve. 

Fig.  12  shows  the  relative  absorption  of  water  and  of  the  solution  of  cal- 
cium chloride  having  a  concentration  of  5.38  normal  and  a  depth  of  20  mm. 
The  corresponding  water-curve  is  marked  throughout  by  the  symbol  H2O. 
The  solution  is  the  more  transparent  from  0.9/jl  to  nearly  Ijul.  The  water  then 
becomes  the  more  transparent  over  a  short  region  of  wave-lengths.  From 
1.05/i  to  1.2]JL  the  solution  is  the  more  transparent.  In  this  region  the  solu- 
tion becomes  as  much  as  25  per  cent  more  transparent  than  the  pure  water, 
as  can  be  seen  by  comparing  the  points  on  the  ''water"  curve  with  the  corre- 
sponding points  on  the  curve  for  the  solution  which  are  vertically  above  the 
points  on  the  water-curve.  The  water  becomes  appreciably  more  trans- 
parent only  at  and  near  the  bottom  of  the  "water-band"  having  a  wave- 
length of  approximately  1/jl.  This  is  the  effect  that  we  would  expect  to  get 
if  the  dissolved  substance  exerted  a  "damping"  effect  on  the  absorption  of 
light  by  water. 

It  will  be  recalled  that  the  salts  which  do  not  form  hydrates  show,  in 
aqueous  solution,  practically  the  same  absorption  as  the  corresponding 
amount  of  water.  It  would,  therefore,  seem  reasonable  to  account  for  the 
differences  in  the  case  of  nonhydrating  and  strongly  hydrating  salts  as  due 
to  the  water  of  hydration,  or  the  water  that,  in  this  case,  is  combined  with 
the  calcium  chloride. 

The  curves  in  fig.  13  are  for  a  smaller  depth  of  the  same  solution  of  cal- 
cium chloride.  This  figure  brings  out  the  same  general  relations  as  was 
shown  in  fig.  12.  The  water-curve  in  the  region  1.25/x  is  above  that  of  the 
solution,  showing  that  water  in  this  region  for  the  shallower  depths  of  solu- 
tion is  more  transparent  than  the  solution.  The  additional  feature  brought 
out  by  this  figure  is  the  water-band  in  the  region  1.4  to  1.5/i.  After  the  first- 
named  water-band  is  passed  the  solution  becomes  more  transparent  than  the 
water  and  remains  so  until  the  wave-length  1.42  is  reached.  Here  both  the 
solution  and  the  water  are  practically  opaque,  as  is  shown  by  both  the  curves 
approaching  the  abscissas. 

The  curve  for  magnesium  chloride  having  a  depth  of  20  mm.  is  almost 
exactly  a  duplicate  of  that  for  calcium  chloride  having  the  same  depth. 
Practically  the  only  difference  worthy  of  mention  is  in  the  region  froml.O/x 
to  l.ljLt.  In  the  case  of  magnesium  chloride  the  water  remains  the  more 
transparent  over  this  region  of  wave-lengths.  In  the  case  of  calcium  chlo- 
ride the  solution  is  the  more  transparent  over  this  region.     The  difference 


54  ABSORPTION   OF  LIGHT  BY   WATER   CHANGED 

in  the  transparency  of  the  water  and  the  solution  throughout  this  region  is, 
however,  not  very  great.  From  l.ljw  towards  the  longer  wave-lengths,  as 
we  come  down  the  descending  arm  of  the  curve  towards  the  second  water- 
band,  the  water  in  the  case  of  the  magnesium  chloride  (as  in  the  case  of  cal- 
cium chloride)  becomes  much  more  opaque  than  the  solution,  the  differences 
here  being  of  the  same  order  of  magnitude  as  those  with  calcium  chloride. 

Fig.  15  gives  the  results  for  magnesium  chloride  with  a  depth  of  layer  of 
1  cm.,  and  the  same  relations  hold  as  in  fig.  14,  for  the  relative  transparency 
of  the  water  and  of  the  solution.  The  water  becomes  the  more  transparent 
from  1.22/x  to  1.34/x.  For  the  longer  wave-lengths  the  solution  becomes 
more  transparent  until  the  region  lAljj,  is  passed.  For  wave-lengths  longer 
than  1.41/x  the  transmission  of  both  solution  and  water  is  practically  zero — 
that  is,  they  both  become  opaque  to  the  longer  wave-lengths. 

The  results  in  fig.  16  bring  out  some  new  features  of  interest  and  impor- 
tance. These  are  the  results  that  were  obtained  with  aluminium  sulphate. 
The  new  feature  shown  by  the  curve  for  aluminium  sulphate,  as  compared 
with  those  for  calcium  chloride  and  magnesium  chloride,  is  that  at  the 
minimum  of  the  curve  corresponding  to  wave-length  Ijjl  the  solution  is 
more  transparent  than  the  corresponding  water.  Beyond  the  wave-length 
1.04/x  the  water  becomes  the  more  transparent  with  aluminium  sulphate  as 
with  magnesium  chloride.  Beyond  the  wave-length  1.1 7/x  the  solution 
becomes  more  transparent  in  this  case  as  with  magnesium  chloride  and 
calcium  chloride. 

If  we  turn  to  fig.  17  the  relations  are  as  follows.  In  the  region  of  1.2/x  the 
water  is  the  more  opaque.  From  1 .29/;t  to  1 .36/x  the  water  becomes  the  more 
transparent.  From  1.36ju  to  the  longest  wave-length  studied,  the  solution 
again  becomes  more  transparent  than  the  corresponding  layer  of  water. 

An  examination  of  all  the  results  thus  far  obtained  bearing  on  this  prob- 
lem leads  us  to  conclude  that  the  greater  transparency  of  the  solution  as 
compared  with  the  water  in  the  solution  must  be  due  to  some  action  of  the 
dissolved  substance  on  the  solvent  water.  The  question  remains,  what  is 
this  action? 

EXPLANATION  OF  THE  RESULTS. 

We  have  seen  from  our  earlier  work  on  the  absorption  spectra  of  solutions, 
which  has  been  in  progress  in  this  laboratory  continuously  for  the  past  eight 
years,  that  the  solvent  can  have  a  marked  effect  on  the  power  of  the  dis- 
solved substance  to  absorb  light.  This  was  first  shown  by  Jones  and 
Anderson,^  and  a  large  number  of  examples  of  this  effect  have  since  been 
found  by  Jones  and  Strong.^  We  interpreted  the  effect  of  the  solvent  on  the 
power  of  the  dissolved  substance  to  absorb  light  as  due  to  a  combination 
between  a  part  of  the  liquid  present  and  the  dissolved  substance.  This 
enabled  us  to  explain  a  large  number  of  facts  which  were  brought  to  light  for 
the  first  time  by  our  investigations  of  the  absorption  spectra  of  solutions. 
Many  of  the  phenomena  which  were  thus  explained,  it  seemed,  could  not  be 

1  Cam.  Inst.  Wash.  Pub.  110.  « Cam.  Inst.  Wash.  Pubs.  130  and  160. 


IN  THE   PRESENCE   OF  STRONGLY  HYDRATED   SALTS.  65 

explained  in  terms  of  any  other  suggestion  that  has  thus  far  been  made. 
In  a  word,  the  solvate  theory  of  solution  as  proposed  by  Jones  about  a  dozen 
years  ago, Ho  supplement  the  theory  of  electrolytic  dissociation  in  order  that 
we  might  have  a  theory  of  the  real  solutions  which  we  use  in  the  laboratory, 
and  not  simply  a  theory  of  ideal  solutions  as  the  theory  of  electrolytic  dis- 
sociation alone  must  be  regarded,  has  served  good  purpose  in  explaining  the 
phenomena  that  have  been  previously  observed  in  connection  with  the 
absorption  of  light  by  solutions  of  dissolved  substances. 

We  are  inclined  to  explain  the  phenomena  recorded  in  this  paper  by  means 
of  the  same  theory.  For  solutions  of  those  substances  which  have  been 
shown  by  entirely  different  methods  not  to  hydrate  to  any  appreciable 
extent,  the  absorption  of  light  by  the  solution  and  by  a  layer  of  water  equal 
in  depth  to  that  of  the  water  in  the  solution,  is  the  same  almost  to  within  the 
limit  of  experimental  error. 

For  those  substances  which  have  been  shown  to  form  complex  hydrates, 
however,  the  absorption  of  light  by  their  solutions  and  by  a  layer  of  water 
equal  in  depth  to  that  of  the  water  in  the  solution  is  very  different.  The 
water  in  these  solutions  is  usually  more  opaque  to  light  than  the  solution — 
or,  in  other  words,  a  solution  is  more  transparent  than  the  water  that  is 
present  in  the  solution. 

The  most  rational  explanation  of  this  phenomenon  appears  to  be  that  the 
part  of  the  water  that  is  combined  with  the  dissolved  substance  has  a  smaller 
power  to  absorb  light  than  pure,  free,  uncombined  water.  The  fact  that 
we  are  able  to  detect  the  difference  between  the  water  in  the  solution  and 
pure  water,  by  its  action  on  light,  we  regard  as  good  evidence  that  water  in 
the  solution  is  different  from  pure,  free  water.  This  difference,  it  seems  to 
us,  can  be  readily  accounted  for  by  the  theory  that  a  part  of  the  water 
present  in  the  solution  is  in  combination  with  the  dissolved  substance. 

We  have  carried  out  similar  investigations  with  aluminium  nitrate,  but  the 
concentration  of  the  strongest  solution  that  could  be  obtained  was  not  suflSi- 
ciently  great  to  show  the  phenomenon  in  question.  We  therefore  do  not 
incorporate  the  results  obtained  with  this  substance.  That  the  solutions 
must  be  very  concentrated  to  show  clearly  the  phenomenon  with  which  we 
are  dealing  is  seen  from  the  results  given  in  table  10.  Here  the  solutions  of 
the  three  salts  in  question  that  were  used  are  more  dilute  than  those  for 
which  the  results  are  tabulated  in  tables  8  and  9.  An  examination  of  table 
10  will  show  that  the  phenomenon  in  question  does  not  manifest  itself  to 
anything  like  the  same  extent  as  with  the  more  concentrated  solutions. 
This  is  exactly  what  we  would  expect  in  terms  of  the  solvate  theory  of  solu- 
tions. The  more  concentrated  the  solution  the  larger  the  total  amount  of 
the  water  present  combined  with  the  dissolved  substance.  If  combination 
between  water  and  the  dissolved  substance  explains  the  facts  recorded  in  this 
paper,  then  the  larger  the  amount  of  water  present  that  is  combined  with 
the  dissolved  substance  the  more  pronounced  the  phenomenon  in  question. 

» Amer.  Caiem.  Joum.,  23,  89  (1900). 


>( 


56  ABSORPTION    OF   LIGHT   BY   WATER   CHANGED 

The  results  obtained  with  aluminium  sulphate  bring  out  the  same  facts 
shown  by  calcium  chloride  and  magnesium  chloride,  and  also  that  water  is 
more  transparent  in  the  region  I.Ijjl  and  more  opaque  at  l/i.  That  the  sul- 
phate should  not  agree  throughout  with  the  chlorides  is  really  not  surprising, 
since  the  sulphates  show  abnormal  results  in  almost  every  particular.  This 
is  probably  due,  in  part  at  least,  to  the  large  amount  of  polymerization 
which  the  sulphate  molecules  in  general  undergo  in  the  presence  of  even 
water  as  a  solvent.  It  should  also  be  remembered  in  the  present  connection 
that  while  calcium  chloride  and  magnesium  chloride  crystallize  with  only  6 
molecules  of  water,  and  are  therefore  only  largely  hydrated,  aluminium  sul- 
phate crystallizes  with  18  molecules  of  water  and  is  therefore  very  largely 
hydrated. 

The  results  in  table  11  are  the  radiomicrometer  deflections  for  a  solution 
of  aluminium  sulphate  and  those  for  water  having  the  same  depth  as  the 
water  in  the  solution  in  question,  and  the  corresponding  data  for  potassium 
chloride.  A  comparison  of  the  two  columns  for  potassium  chloride  and  its 
corresponding  water  shows  that  the  two  are  almost  equally  transparent  to 
all  the  wave-lengths  studied. 

A  comparison  of  the  aluminium  sulphate  with  its  corresponding  water 
brings  out  the  phenomenon  that  we  are  now  discussing  in  a  very  pronounced 
manner. 

One  other  relation  of  a  general  character  should  be  pointed  out.  The 
curves  (figs.  12  to  17)  show  that  the  addition  of  salt  to  water  shifts  the 
absorption  towards  the  longer  wave-lengths.  This  is  analogous  to  what  had 
already  been  found  by  Jones  and  Uhler,^  Jones  and  Anderson,^  Jones  and 


70 


60 


50- 


40- 


30 


20 


H20 

X^^^^^^^\ 

1 

VhzO^^^ 

\ 

• 

H^oU 

CaCl2,5.38N. 

\\ 

r                 Depth  2  cm. 

r                                     1 

v= 

0.9  1.0  1.1  1.2 

Fia.  12. 
»  Cam.  Inst.  Wash.  Pub.  60.  *  Cam.  Inst.  Wash.  Pub.  110. 


IN   THE   PRESENCE   OF   STRONGLY  HYDRATED   SALTS.  57 

Strong/  and  Guy  and  Jones,^  when  the  absorption  of  salts  as  affected  by  the 
water  present  was  studied.  It  was  found  that  rise  in  temperature  and 
increase  in  the  concentration  of  the  solution  both  tended  to  shift  the  ab- 
sorption of  the  salt  towards  the  longer  wave-lengths.  The  effect  of  rise  in 
temperature  and  the  increase  in  the  concentration  of  the  solution  tended  to 
simplify  the  hydrates  in  combination  with  the  particles  of  the  salt.  The 
resonator  within  this  simplified  system  seems  to  vibrate  so  as  to  shift  the 
absorption  bands  towards  the  red. 

The  effect  of  the  salt  on  the  absorption  of  the  water  is  the  same  as  that  of 
rise  of  temperature  and  increase  of  concentration  on  the  absorption  of  the 
dissolved  substance.  We  would  naturally  look  for  a  similar  explanation  of 
the  two  sets  of  phenomena.  It  has  been  suggested  by  Dr.  Guy,  that  the 
effect  of  the  salt  on  the  absorption  of  light  by  water  may  be  due  to  the 
breaking  down  of  the  associated  molecules  of  water  by  the  dissolved  sub- 
stance. This  would  be  in  keeping  with  the  fact  established  by  Jones  and 
Murray,^  that  one  associated  substance  when  dissolved  in  another  associated 
substance  diminishes  its  association. 

In  terms  of  this  explanation,  however,  it  is  a  little  difficult  to  see  why  non- 
hydrated  salts,  such  as  were  used  in  this  work,  do  not  also  diminish  the  asso- 
ciation of  water  and  cause  a  shifting  of  its  absorption  bands  towards  the 
longer  wave-lengths.  It  may  be  that  the  effect  of  the  dissolved  substance 
in  breaking  down  the  association  of  the  water  is  pronounced  only  in  the  case 
of  water  of  hydration  or  the  water  that  is  combined  with  the  dissolved  sub- 
stance, and  that  the  explanation  offered  above  is  fundamentally  correct. 


1^ 


» Cam.  Inst.  Wash.  Pubs.  130  and  160.  » Amer.  Chem.  Journ.,  30, 193  (1903), 

2  Amer.  Chem.  Journ.,  49,  1  (1913). 


58 


ABSORPTION   OF  LIGHT   BY  WATER   CHANGED 


80 

H2O 
\ 

/;?= 

""^ 

70 

I        \ 

Hit)/' 

\ 

60 

\ 

// 

Vi 

50 

V 

y/ 

\\ 

40 

HzOU 

30 

MgCl2.4.96N. 
Depth  2cm. 

^ 

20 

0.9 


1.0 


1.2 


Fig.  U. 


MgCl2,4.96N 
Depth  1cm. 


IN   THE    PRESENCE    OF   JSTKONGLY    HYDRATED    SALTS. 


59 


80 

70 

60 

50- 

40 

30- 

20- 


HzO 


Al2(S04)3,I.OI7N. 
Depth  2cm. 


0.9 


I.O 


1.2 


Fig.  16. 


1.15 


Al2(S04)3,I.OI7N 
Depth  1cm. 

i!2 


1.5 


Fio.  17. 


CHAPTER  VI. 

ABSORPTION  SPECTRA  OF  A  NUMBER  OF  SALTS  AS  MEASURED 
BY  MEANS  OF  THE  RADIOMICROMETER. 

The  results  tabulated  and  discussed  in  Chapters  IV  and  V,  which  are  con- 
cerned with  the  energy  measurements  of  the  absorption  spectra  of  solutions 
by  means  of  the  radiomicrometer,  were  made  by  comparing  the  intensity  of  a 
given  source  of  light  (after  passing  through  the  solution)  with  the  intensity 
of  the  same  source  of  light  after  passing  through  an  equal  depth  of  water. 
In  a  word,  the  depths  of  cells  in  each  case  were  the  same.  As  has  already 
been  stated,  a  cell  whose  depth  was  1  cm.  was  filled  with  the  solution  and 
placed  in  the  path  of  the  beam  of  light  and  the  deflection  of  the  instrument 
noted;  then  a  cell  of  the  same  depth  was  filled  with  the  solvent  and  interposed 
in  exactly  the  same  position  as  the  former  cell,  and  the  deflection  of  the 
instrument  again  noted.  Denoting  the  former  by  I  and  the  latter  by  7o  we 
get  the  ratio  I/Io,  which  represents  the  percentage  transmission  of  the  solu- 
tion as  compared  with  water.  Such  a  procedure  was  repeated  at  frequent 
intervals  throughout  the  spectrum,  locating  a  series  of  points  through 
which  the  transmission  curves  could  be  drawn. 

Certain  phenomena  presented  themselves  throughout  the  course  of  this 
investigation,  which  suggested  a  more  careful  study  of  some  of  the  absorp- 
tion bands  located  in  the  infra-red  portion  of  the  spectrum;  and  at  the  same 
time  it  was  thought  advisable  to  map  the  absorption  spectra  of  some  of  the 
more  common  salts  of  cobalt,  nickel,  etc.,  in  terms  of  Beer's  law;  since  up  to 
the  time  of  this  investigation  no  satisfactory  quantitative  study  of  the 
infra-red  spectrum  of  these  salts  had  appeared. 

In  order  to  make  a  careful  study  of  the  exact  intensity  of  the  various  por- 
tions of  any  given  bands,  it  is  clear  that  we  are  dealing  with  a  much  more 
complex  and  intricate  problem  than  simply  with  the  location  of  the  band; 
and  on  this  account  it  was  necessary  to  improve  our  apparatus  and  at  the 
same  time  to  exert  more  care,  if  possible,  in  carrying  out  any  given  operation. 

It  was  early  found  that  if  we  desired  to  study  that  region  of  the  infra-red 
spectrum  in  which  water  had  considerable  absorption,  we  must  not  compare 
our  solutions  with  an  equal  depth  of  layer  of  water,  as  noted  above;  but  with 
a  depth  of  layer  equal  to  the  water  in  the  solution,  which  in  the  most  con- 
centrated solutions  was  much  less  than  the  actual  depth  of  the  cell  containing 
the  solution — a  part  of  the  cell's  depth  being  occupied  by  the  dissolved  sub- 
stance. Even  when  such  a  correction  was  made,  it  was  found  that  for  a 
given  wave-length,  in  the  water  absorption  bands,  the  solution  gave  greater 
deflections  than  did  the  solvent,  i.  e.,  that  in  such  regions  the  solution  was 
actually  more  transparent  than  an  equivalent  depth  of  water. 

61 


62  ABSORPTION    SPECTRA   OF   A    NUMBER    OF   SALTS 

Kemembering  that  the  solutions  with  which  we  were  then  working,  i.  e., 
solutions  of  salts  of  neodymium  and  praseodymium,  were  strongly  hydrated, 
it  was  thought  that  in  view  of  the  fact  that  at  least  a  part,  and  in  the  con- 
centrated solutions  a  considerable  part  of  the  water  present  was  there  as 
water  of  hydration,  it  would  be  advisable  to  study  the  effect  of  colorless 
hydrated  salts  upon  the  absorption  of  water. 

This  chapter  of  our  work  has  been  sufficiently  discussed  elsewhere  in  this 
monograph,  and  will  be  taken  up  here  only  to  state  that  these  experiments 
showed  clearly  that  there  were  many  variables  to  be  considered.  We  have, 
first,  the  effect  of  the  solvent  on  the  absorption  of  the  solute ;  and,  secondly, 
the  effect  of  the  solute  upon  the  absorption  of  the  solvent.  In  addition  to 
these,  there  was,  of  course,  the  absorption  of  the  solvent  and  the  solute  inde- 
pendently. Such  being  the  case,  we  would  not  be  obtaining  comparable 
results  for  various  dilutions  of  any  solutions  in  terms  of  Beer's  law,  even  if 
we  did  compare  each  dilution  with  an  equivalent  amount  of  water.  It  is 
clear  that  by  so  doing  we  would  not  be  getting  comparable  ratios,  since  the 
solvent  and  the  solute  were  mutually  affecting  each  other's  absorption;  and 
this  effect  would  not  be  the  same  for  the  different  dilutions  of  the  same  salt. 

MODE  OF  PROCEDURE. 

It  is,  however,  possible  to  get  the  exact  transmission  of  a  given  depth  of 
solution  by  a  method  of  differentiation.  If  we  placed  in  cell  ^11  mm.  of  a 
solution  and  in  cell  B  1  mm.  of  the  same  solution,  the  ratio  representing  the 
respective  deflections  of  the  instrument,  when  these  cells  are  alternately 
placed  in  the  path  of  the  beam  of  light,  should  give  the  absorption  or  trans- 
mission of  (11  —  1)  or  10  mm.  of  the  solution. 

Since,  if  we  let  A  be  the  percentage  absorption  of  a  unit's  depth  of  layer  of 
the  solution,  and  Jo  the  initial  intensity  of  the  light  impinging  upon  the  sur- 
face, we  get 

A/o  =  amount  of  light  absorbed  by  first  unit  layer  of  the  solution. 
Then, 

Iq—IqA  -  7o(l  —  ^)  =  light  incident  upon  surface  of  second  unit  layer. 
Denoting  this  by  /i,  we  get 

h  =  h-IoA=Io{l-A)  OT^  =  l-A 

Considering  again  the  third  unit  layer,  we  get,  by  similar  reasoning, 

Ii—IiA  =  amount  of  light  incident  upon  its  surface. 

Denoting  this  by  hj  we  get 

J2=/i-7iA=7i(l-A) 

but  Ii  =  Io{l-A);  therefore,  /2  =  7o(l-A)2;  hence  7„  =Io(l-Af.  We  can 
then,  by  this  process,  obtain  transmissions  for  given  depths  of  solution 
and  for  varying  concentrations.  This  was  the  method  adopted  throughout 
this  chapter  of  the  work. 


AS   MEASURED   BY  MEANS   OF   THE   RADlOMICROMETER.  63 

DESCRIPTION  OF  CELLS  USED. 

In  all  cases  where  we  were  dealing  with  different  depths  of  layer,  it  was 
necessary  to  use  cells  adjustable  in  length.  A  very  satisfactory  form  of  cell 
was  devised  and  used  throughout  the  latter  part  of  this  work.  It  consisted 
essentially  of  two  brass  cylinders  telescoping  neatly  into  each  other.  The 
external  diameter  of  the  outside  cylinder  was  about  2|  inches,  and  the  thick- 
ness of  the  walls  was  in  every  case  about  2  mm.,  which  was  sufficient  to  with- 
stand handling  without  danger  of  changing  the  shape  of  the  cell.  Into  the 
ends  of  each  cylinder  there  was  sealed,  by  means  of  Wood's  metal,  a  glass 
plate  about  1  mm.  thick,  made  of  the  very  best  optical  glass.  In  all  cases 
the  glass  plates  were  so  nearly  parallel  as  to  show  interference  fringes ;  and 
both  cells  gave  the  same  deflections,  either  when  empty  or  filled  with  the 
same  solution  and  placed  in  the  path  of  the  light  before  the  radiomicrometer. 

After  adjusting  the  glass  ends  and  fixingthemsecurely  by  meansof  Wood's 
metal,  the  entire  cell  was  first  plated  with  silver,  being  taken  out  of  the 
plating-bath  from  time  to  time  and  polished  to  a  bright  surface  with  the 
finest  crocus  paper.  On  top  of  this  silver  coating  a  heavy  plating  of  gold 
was  deposited.  The  distance  between  the  glass  plates  fastened  to  the  ends 
of  the  telescoping  cylinders,  which  determined  the  depth  of  layer  of  solution 
used,  was  in  all  cases  fixed  by  gold-plated  washers,  whose  thickness  had  been 
accurately  measured  to  0.001  inch  by  means  of  a  vernier  caliper. 

Before  any  series  of  readings  was  made,  the  positions  of  the  two  cells  was 
so  adjusted  in  the  sliding  carriage  as  to  give  equal  deflections,  when  alter- 
nately placed  in  the  same  position  before  the  radiomicrometer,  in  that  part 
of  the  spectrum  where  neither  the  solute  nor  solvent  had  any  absorption; 
and  from  time  to  time  throughout  the  experiment  duplicate  readings  were 
made  on  this  point  to  see  that  the  cells  had  not  changed  their  relative 
positions.  In  case  any  change  was  noted,  a  duplicate  series  of  readings  was 
always  made.  Such  readings  upon  the  same  cell  usually  agreed  to  about 
one  division  of  the  scale,  which  corresponded  to  about  1  to  2  per  cent, 
depending  upon  the  throw  of  the  instrument.  In  the  midst  of  the  very 
intense  absorption  bands,where  the  deflections  of  the  instrument  were  small, 
reaching  zero  at  many  points,  the  error  resulting  from  any  drift  in  the  instru- 
ment or  reading  of  the  scale  was  greater  than  the  mean  error  given  above. 

In  nearly  all  cases  new  solutions  were  made  up  and  the  results  duplicated, 
so  that  the  tables  and  curves  below  represent  a  mean  of  several  series  of 
readings.  In  most  cases  the  agreement  was  very  satisfactory,  usually  the 
difference  not  being  over  3  per  cent. 

Since  any  change  in  the  position  of  the  prism  was  a  determining  factor  in 
the  portion  of  the  spectrum  which  fell  upon  the  thermo-j  unction,  and  since 
in  the  very  intense,  sharp  bands  of  the  neodymium  salts  any  slight  change  in 
the  position  of  the  prism  would  make  a  great  difference  in  the  final  results, 
great  care  had  to  be  exerted  in  setting  the  head  reading  of  the  spectroscope. 
Such  difficulties  were  not  met  with  in  solutions  where  the  absorption  bands 
were  broad  and  diffuse,  as  in  salts  of  cobalt,  nickel,  etc. 


64  ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 

In  studying  the  changes  which  might  occur  in  any  band,  it  is  of  course 
necessary  that  all  conditions  be  as  nearly  as  possible  the  same.  One  of  the 
most  important  factors  here  is  that  of  the  width  of  the  slits  of  the  spectro- 
scope. With  those  solutions  whose  absorption  bands  are  broad  and  diffuse, 
not  having  such  well-defined  edges  as  with  the  salts  of  neodymium  and 
praseodymium,  this  is  not  such  a  determining  factor.  Should  the  band  be 
very  narrow — say  approaching  that  of  the  width  of  the  slit  necessary  to  be 
used  in  order  to  secure  reasonable  deflections  of  the  instrument — it  is  seen 
that  any  slight  change  in  the  slit  will  make  a  large  difference  in  the  amount 
of  light  falling  on  the  thermal-junction. 

Considering  a  concrete  example,  let  us  suppose  that  the  slit-width  is  just 
equal  to  that  of  the  absorption  band,  under  a  given  dispersion.  If,  now,  the 
band  and  the  slit  exactly  coincide,  it  is  evident  that  no  light  will  be  falling 
upon  the  junction,  this  being  indicated  by  zero  deflection  of  the  instrument. 
If,  on  the  other  hand,  the  slit  is  slightly  wider  than  the  band,  some  light  will 
enter  around  the  edges  of  the  band;  and,  though  the  narrow  band  may  act- 
ually have  complete  absorption  at  a  given  point,  it  would  not  be  indicated  by 
the  instrument,  since  some  Ught  is  entering  around  the  edges  of  the  band. 

Denoting  the  deflection  of  the  instrument  for  a  cell  of  2  mm.  depth  of  a 
solution  of  X  concentration  by  A,  and  the  same  for  1  mm.  of  the  same  solution 
by  B,  we  get,  by  the  differential  method  discussed  above,  the  ratio  A/B  for 
the  intensity  of  the  light  transmitted  by  (2  —  1)  or  1  mm.  of  the  solution  in 
question. 

By  a  similar  reasoning  we  get  the  ratio  A' jB'  for  the  value  of  the  trans- 

mission  of  a  solution  of  concentration —,  using  absorbing  layers  21  mm.  and 

1  mm.,  respectively.  While  such  a  method  is  theoretically  and  mathe- 
matically correct  for  infinitely  narrow  slit-widths,  and  practically  so  for 
bands  which  are  wide  in  comparison  with  the  necessary  slit-widths,  yet  in 
the  case  of  very  sharp,  narrow  neodymium  bands  it  has  been  found  not  to 
give  comparable  results.  The  reason  for  this  is  clearly  seen  in  the  light  of 
the  facts  discussed  above. 

Let  us  consider  the  ratios  AjB  and  A' /B'.  In  the  first  case  we  are  deal- 
ing with  concentrated  solutions,  where  the  absorption  bands  are  broad; 
hence  B  is  small,  and,  in  case  the  slit-width  is  comparable  with  the  width  of 
the  absorption  band,  B  will  be  very  much  smaller  than  J5',  since  B'  is  only 

1  mm.  of  an— ■  concentration  solution.     In  a  word,  B,  which  is  1  mm.  of  the 

more  concentrated  solution,  has  20  times  the  number  of  absorbers  as  has  an 
equal  depth  represented  by  B' ,  and  a  decrease  in  the  denominator  of  the 
fraction  means  an  increase  in  its  value. 
While  the  ratios  A/B  and  A'/B'  give  the  transmissions  for  1  mm.  of  a 

a; 
solution  of  concentration  x,  and  20  mm.  of  a  solution  of  concentration  — 

respectively,  provided  the  slits  are  narrow;  yet  in  the  visible  part  of  the 


AS   MEASURED   BY  MEANS   OF   THE  RADlOMICROMETER.  66 

spectrum,  where  such  wide  slits  had  to  be  used  on  account  of  the  small 
amounts  of  energy  in  this  region,  these  ratios  are  not  comparable. 

For  this  reason  we  have  confined  the  larger  part  of  our  work  on  neo- 
dymium  compounds  almost  entirely  to  wave-lengths  greater  than0.7/z.  In 
all  the  following  tables  and  curves  representing  these  data,  constant  slit- 
widths  of  0.2  mm.  have  been  used.  This  was  the  minimum  width  which 
could  be  employed,  in  order  to  get  reasonable  deflections  throughout  the 
spectrum  from  0.7/x  to  1/x.  Experiments  have  shown  that  any  error  result- 
ing from  slit-widths  would  not  amount  to  more  than  3  or  4  per  cent  through- 
out this  region. 

The  source  of  light  was,  as  in  the  previous  chapters,  a  Nernst  glower  carry- 
ing about  1.2  amperes,  and  the  current  kept  constant  by  means  of  an  adjust- 
able slide-wire  resistance.  The  source  of  current  was  a  large  number  of 
storage  cells,  and  this  was  never  allowed  to  vary  over  0.01  ampere.  Great 
care  was  exerted  in  keeping  the  current  constant  while  obtaining  a  single 
ratio,  since  this  is  really  the  only  time  in  which  a  slight  change  in  current 
density  was  dangerous. 

DISCUSSION  OF  TABLES  AND  CURVES. 
NEODYMIUM  CHLORIDE  IN  WATER. 

Table  12  gives  the  observed  transmissions  of  solutions  of  neodymium 
chloride  in  water.  In  all  the  tables  the  following  four  dilutions  have  been 
studied,  the  depths  of  cell  being,  generally,  2.5,  5, 10,  and  20  mm.,  respec- 
tively; and  the  concentrations  being  made  so  as  to  keep  nXd  =  k.  In 
column  1  of  each  table  there  is  given  X,  taken  at  such  intervals  as  the  solu- 
tion in  question  required.  In  those  portions  of  the  spectrum  where  the 
transmission  was  complete,  or  very  nearly  so,  these  intervals  were  greater 
than  in  those  regions  where  there  were  absorption  bands. 

Reading  from  left  to  right  in  this  table,  beginning  with  column  2,  there  are 
given  the  absorptions  for  solutions  of  the  following  concentrations:  2.141, 
1.07,  0.535,  and  0.267  normal,  respectively;  the  corresponding  depths  of 
absorbing  layer  being  2.5,  5,  10,  and  20  mm.,  respectively.     In  every  case 

the  transmission  was  obtained  from  the  ratio  — - — ,  where  x  is  2.5,  5,  10, 

and  20  mm.,  respectively. 

In  all  cases  the  concentrated  mother-solution  was  carefully  made  up,  its 
concentration  determined  by  a  gravimetric  precipitation  of  the  metal ,  and  the 
succeeding  solutions  made  by  diluting  measured  parts  of  the  mother-solution. 

Observations  are  given  here  over  only  that  portion  of  the  infra-red  spec- 
trum from  X6800  to  XIOOOO.  It  is  in  this  region  that  the  most  pronounced 
neodymium  bands  occur.  It  was  thought  advisable  not  to  go  further  into 
the  infra-red,  since  beyond  Ijjl  the  general  absorption  due  to  the  solvent  is 
very  marked.  This  would,  of  course,  interfere  with  a  quantitative  study  of 
any  band  occurring  in  this  region,  since  it  is  impossible  to  separate  the  two 
absorptions,  previous  work  having  shown  that  they  are  not  additive. 


66 


ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 


The  work  in  the  visible  region  of  the  spectrum  was  limited  by  the  slit- 
widths  necessary  to  be  used,  which  has  been  mentioned  and  discussed  above. 
We  have  rather  chosen  a  limited  portion  of  the  infra-red,  over  which  we 
could  work  without  altering  either  the  intensity  of  the  light  or  the  slit-width, 
which  was  in  all  cases  0.2  mm. 

The  curves  representing  table  12  are  given  in  figs.  18  to  21  inclusive. 
The  percentages  of  transmission  are  plotted  as  ordinates,  while  the  wave- 
lengths are  given  as  abscissae.  An  examination  of  these  curves  shows  three 
pronounced  minima,  representing  the  three  absorption  bands,  with  their 
centers  near  X7300,^  X7950,  and  X8700,  and  less-marked  bands  near  X7150 
and  X9000.     The  latter  of  these  small  bands  is  possibly  due  in  part  to  the 

Table  12. — Percentage  Transmission  oj  Neodymium  Chloride  Solutions. 


X 

D.=2.5mm. 

D.=5mm, 

D.  =  10min. 

D.=20mm. 

X 

D.=2.5mm. 

D.=5mm. 

D.  =  10mm. 

D.=20inm 

C.=2.141N. 

C.  =  1.071. 

C.  =0.635. 

C. =0.267. 

C.=2.141N. 

C.  =  1.071. 

C. =0.535. 

C.  =0.267. 

686 

93 

88 

88 

86 

800 

15 

10 

8 

6 

693 

95 

95 

95 

94 

805 

24 

23 

22 

18 

698 

96 

96 

96 

94 

809 

40 

39 

38 

37 

704 

96 

96 

98 

94 

814 

58 

58 

58 

53 

708 

96 

93 

95 

92 

819 

80 

78 

78 

76 

712 

92 

93 

95 

88 

825 

89 

91 

91 

82 

716 

88 

89 

88 

85 

830 

93 

92 

93 

88 

720 

81 

78 

81 

81 

834 

94 

95 

95 

88 

723 

64 

62 

63 

56 

839 

93 

93 

93 

87 

726 

32 

31 

25 

23 

845 

91 

92 

91 

86 

730 

7 

7 

5 

6 

850 

87 

86 

87 

78 

733 

0 

0 

0 

0 

856 

75 

73 

71 

66 

737 

0 

0 

0 

0 

861 

54 

46 

43 

40 

741 

5 

2 

1 

2 

867 

29 

21 

18 

16 

746 

18 

14 

12 

5 

!  872 

28 

24 

23 

18 

751 

36 

39 

29 

28 

1  877 

40 

39 

40 

34 

755 

54 

55 

52 

49 

\    882 

53 

48 

52 

47 

759 

75 

74 

72 

68 

i  888 

60 

59 

60 

53 

763 

85 

85 

83 

81 

!  894 

Gl 

59 

61 

56 

767 

84 

86 

85 

83 

!  900 

67 

66 

66 

59 

770 

79 

81 

80 

76 

906 

80 

78 

79 

71 

774 

67 

73 

65 

61 

912 

90 

92 

89 

79 

779 

47 

45 

45 

40 

917 

96 

93 

94 

86 

783 

29 

27 

20 

24 

923 

98 

97 

96 

86 

787 

22 

10 

10 

7 

1  928 

98 

90 

96 

84 

791 

0 

0 

0 

0 

i  933 

98 

96 

96 

81 

796 

0 

0 

0 

0 

938 

98 

94 

90 

75 

absorption  of  the  solvent;  but  since  its  intensity  does  not  increase  markedly 
with  dilution,  it  is  more  probably  a  doublet.  Considering  the  curves  repre- 
senting all  four  dilutions,  we  see  that  the  X7300  and  X7900  bands  show  com- 
plete absorption  over  a  considerable  range  of  wave-lengths,  and  any  change 
in  intensity  could  not  be  very  noticeable.  The  X8700  band,  however,  has 
its  minima  gradually  lowered  as  we  pass  from  curve  18  to  curve  21,  ^.  e.,  in 
the  direction  of  increasing  dilution.  This  phenomenon  has  been  noted  else- 
where in  this  monograph,  and  a  possible  explanation  of  it  based  upon  a 
theory  of  resonance  suggested.  A  closer  and  more  exact  study  has  shown 
that,  although  the  phenomenon  is  a  real  one,  yet  it  is  probable  that  it  may 

1  The  wave-lengths  in  the  above  and  following  tables  are  given,  in  general,  to  only 
three  places. 


AS   MEASURED   BY   MEANS   OF   THE   RADIOMICROMETER. 


67 


in  part  be  due  to  the  combined  effect  of  the  sUght  water  absorption  and, 
even  a  more  important  factor,  the  sHt-widths,  as  discussed  above. 

The  regions  of  maximum  transmission  occur  near  X7600  and  X8400,  and 
solutions  of  neodymium  chloride  become  almost  completely  transparent 
beyond  1/x,  except  for  the  general  absorption  of  the  solvent.  Slight  absorp- 
tion bands  occur  in  this  region,  one  near  1.5/x,  but  they  are  so  masked  by  the 
intense  water  absorption  that  it  was  found  impossible  to  make  a  quanti- 
tative study  of  them. 


lOu 


75 


50 


25 


Neodymium  Chloride 

Cell  Depth     2.5mm. 
Concentration     2.141  N, 


0.65/i  0.7// 


0,75/^  0.8a 

Fig.  18. 


0.85/u 


OSju 


].M 


)00 


75 


50 


25 


0.65// 


Neodymium  Chloride 

Cell  Depth     5i 
Concentration     1.071  N. 


O  75//  0.8// 

Fig.  19. 


0,95// 


From  a  comprehensive  study  of  the  four  curves  representing  the  absorp- 
tion spectra  of  concentrated  solutions  of  neodymium  chloride,  it  seems 
probable  that  Beer's  law  holds  quantitatively  for  the  infra-red  region,  except 
for  such  slight  changes  as  have  been  fully  discussed  above. 


68 


ABSORPTION   SPECTRA   OF  A   NTTMBER  OF  SALTS 


Since,  as  mentioned  above,  the  absorption  bands  with  their  centers  near 
X7300  and  X7950,  in  solutions  of  such  concentrations  as  are  given  in  table  12, 
reach  complete  absorption,  and  as  in  such  cases  it  would  not  be  easy  to 


100 


§75 


25 


0.65a 


Neodymium  Chloride 

Cell  Depth     iOmm. 
Concentration    0.535N. 


0.75a  0.8/i 

Fig.  20. 


0.95a 


100 


3   75 

5 


O 

;2   50 

z 


25- 


0.65,^ 


Neodymium  Chloride 

Cell  Depth     20mm. 
Concentration     0.267  N. 


0.95// 


detect  any  change  in  their  intensity,  it  was  thought  advisable  to  make  a 
study  of  a  series  of  more  dilute  solutions  of  the  same  salt. 

The  results  of  this  experiment  are  given  in  table  13.     Reading  from  left  to 
right,  the  concentrations  are  0.536,  0.267,  0.133,  and  0.067  normal,  respec- 


AS   MEASURED   BY  MEANS   OF   THE   RADIOMICROMETER. 


69 


tively,  the  corresponding  depths  of  absorbing  layers  being  2.5,  5,  10,  and 
20  mm.,  respectively.  Here  again  it  is  seen  that  the  conditions  of  Beer's 
law  are  adhered  to.  The  results  are  graphically  represented  in  figs.  22  to  25, 
inclusive.    These  curves  show  minima  in  about  the  same  positions  as  did  the 


100 


3   75 


?i  50 


25 


Neodymium  Chloride 

Cell  Depth     2.5mm. 
Concentration    0.536  N. 


0.65// 


0.7/u 


0.75//  0.8m 

FiQ,  22. 


0.85a 


0.9m 


0.95// 


100 


75 


50 


25 


Neodymium  Chloride 


Cell  Depth    5mm. 
Concentration     0.267N. 


0.65// 


0.7// 


0.75// 


0.8// 

Fig.  23. 


0.85yU 


0.^ 


0.95/f 


curves  representing  the  more  concentrated  solutions,  but  since  the  solutions 
are  more  dilute,  they  are  accordingly  more  transparent;  hence  the  minima 
in  the  curves  are  not  so  pronounced.  The  maximal  absorption  occurring 
near  X7300  and  X7900  are  at  about  25  per  cent. 


70 


ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 


It  will  be  noticed  that  beyond  O.QjU  all  of  the  curves  drop  sharply  with 
dilution,  which  is  due  entirely  to  the  increasing  absorption  of  the  water. 

Figs.  22  to  25,  inclusive,  show  just  what  might  be  anticipated  from 
figs.  18  to  21,  a  lowering  of  absorption  maxima  as  we  pass  towards  the 
more  dilute  solutions.    This  change  is  most  marked  in  the  X8700  band,  and 


100 


75 


^   50 

(J 

< 

z 

UJ 

Ql 

25 


Neodymium  Chloride 

Cell  Depth   10  mm. 
Concentration  0.133  N 


0.65;/ 


0.7/^ 


0.75//  0.8m 

Fig.  24. 


0.85// 


0.9// 


0.95// 


100 


75 


50 


25 


Neodymium  Chloride 

Cell  Depth  20  mm. 
Concentration  0.067  N 


0.65// 


0.7/i 


0.75/i 


0.8^ 

Fig.  25. 


0.85// 


0.9m 


0S5// 


it  is  in  this  region  that  the  absorption  of  the  water  is  most  pronounced, 
although  a  20  mm.  layer  of  water  in  this  region  has  at  no  point  over  10  per 
cent  absorption.  The  change  in  the  intensity  of  the  absorption  band  is 
greater  than  this  amount,  but  it  seems  probable  that  this,  together  with  the 
added  correction  for  the  slit-widths,  may  account  for  the  phenomenon,  and 
that  Beer's  law  holds  for  the  dilute  solutions  of  neodymium  chloride. 


AS   MEASURED   BY   MEANS   OF   THE   RADIOMICROMETER. 


71 


Table  13. — Transmission  of  Neodymium  Chloride  Solutions  (Dilute). 


-V   D.  =  2.5mm. 
^   C.=0.536N. 

1 

D.=5mm. 

D.  =  10mm. 

D.=20mm. 

X 

D.=2.5min. 

D.=5mm. 

D.  =  10mm. 

D.=20mm. 

C.=0.267N. 

C.=0.133N. 

C.=0.067N. 

C.=0.536N. 

C.=0.267N. 

C.=0.133N. 

C.=0.067N. 

686 

92 

95 

94"" 

92 

800 

45 

39 

38 

39 

693 

93 

95 

95 

97 

805 

64 

59 

57 

57 

698 

97 

95 

95 

97 

809 

78 

72 

71 

70 

704 

98 

95 

96 

97 

814 

85 

84 

82 

82 

708 

98 

95 

97 

98 

819 

92 

91 

88 

88 

712 

100 

98 

96 

96 

825 

100 

94 

94 

94 

716 

100 

98 

98 

92 

830 

100 

96 

94 

93 

720 

96 

94 

89 

89 

834 

100 

94 

94 

92 

723 

85 

84 

83 

84 

839 

100 

96 

93 

91 

726 

59 

54 

58 

53 

845 

98 

95 

94 

91 

730 

42 

41 

40 

34 

850 

97 

94 

90 

89 

733 

31 

29 

26 

25 

856 

92 

87 

84 

83 

737 

25 

24 

20 

22 

861 

78 

77 

74 

70 

741 

28 

29 

29 

29 

867 

70 

64 

62 

60 

746 

47 

44 

51 

44 

872 

64 

60 

59 

56 

751 

69 

65 

65 

60 

877 

74 

71 

68 

66 

755 

85 

82 

82 

78 

882 

84 

80 

76 

75 

759 

93 

86 

90 

86 

888 

89 

84 

82 

78 

763 

97 

90 

92 

92 

894 

90 

84 

83 

79 

767 

97 

95 

90 

92 

900 

91 

88 

86 

81 

770 

97 

93 

90 

89 

906 

95 

90 

87 

83 

774 

91 

86 

87 

84 

912 

98 

95 

90 

84 

779 

79 

76 

78 

74 

917 

100 

96 

92 

85 

783 

69 

61 

59 

56 

923 

100 

96 

93 

84 

787 

47 

39 

38 

34 

928 

100 

95 

92 

84 

791 

23 

24 

20 

18 

933 

100 

94 

94 

83 

796 

28 

25 

23 

23 

938 

100 

94 

95 

78 

Table  14. — Transmission  of  Neodymium  Nitrate  Solutions. 


X 

D.=2.5mm. 

D.=5mm. 

D.  =  10mm. 

D.=20mm. 

X 

D.=2.5min. 

D.=5mm. 

D.  =  10mm. 

D.=20mm. 

C.=2.010N. 

C.  =  1.05  N. 

C.=0.525N. 

C.=0.262N. 

C.=2.010N. 

C.=1.05  N. 

C.=0.525N. 

C.«0.262N. 

686 

86 

88 

85 

78 

800 

12 

10 

6 

6 

693 

93 

96 

91 

89 

805 

21 

19 

14 

15 

698 

93 

96 

93 

93 

809 

35 

33 

30 

30 

704 

96 

94 

93 

91 

814 

49 

49 

47 

47 

708 

95 

96 

96 

88 

819 

63 

67 

65 

64 

712 

89 

90 

94 

89 

825 

79 

80 

79 

77 

716 

84 

88 

88 

82 

830 

82 

88 

89 

82 

720 

78 

79 

78 

78 

834 

92 

92 

93 

85 

723 

80 

63 

61 

58 

839 

96 

94 

94 

89 

726 

40 

33 

30 

28 

845 

92 

91 

91 

85 

730 

14 

14 

10 

9 

850 

87 

84 

85 

77 

733 

0 

0 

0 

0 

856 

76 

70 

72 

65 

737 

0 

0 

0 

0 

861 

61 

57 

63 

47 

741 

11 

4 

0 

0 

867 

43 

36 

35 

32 

746 

12 

15 

6 

6 

872 

30 

27 

26 

22 

751 

22 

23 

22 

17 

877 

31 

30 

28 

25 

755 

38 

39 

40 

38 

882 

43 

42 

41 

36 

759 

60 

60 

62 

55 

888 

55 

52 

51 

48 

763 

75 

75 

74 

70 

894 

65 

60 

59 

61 

767 

80 

80 

83 

79 

900 

68 

66 

66 

62 

770 

75 

75 

79 

75 

906 

77 

75 

75 

67 

774 

64 

66 

68 

62 

912 

86 

84 

83 

75 

779 

47 

47 

48 

44 

917 

94 

90 

97 

83 

783 

32 

32 

29 

26 

923 

98 

92 

90 

84 

787 

16 

10 

13 

9 

933 

100 

96 

88 

86 

791 

11 

8 

2 

5 

938 

100 

96 

83 

77 

796 

0 

0 

0 

0 

72 


ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 
NEODYMIUM  NITRATE. 


Table  14  gives  the  percentage  transmission  for  solutions  of  neodymium 
nitrate.  Column  1  gives  the  respective  wave-lengths  at  such  intervals  as 
the  solutions  required.  Reading  from  left  to  right,  we  find  the  percentage 
transmissions  for  the  following  concentrations:  2.010,  1.050,  0.525,  and 
0.262  normal,  respectively,  the  corresponding  depths  of  absorbing  layers 


100 


75 


50 


25 


0.65yW 


Neodymium  Nitrate 

Cell  Depth  2.5  mm. 
Concentration  2.01  N 


0.75/U  0.8/x 

Fig.  26. 


0.95/i 


100 


75 


50 


25- 


Neodymium  Nitrate 

Cell  Depth  5  mm. 
Concentration  1.05  N 


0J5$M 


0.7a 


0.75// 


0.8/a 

Fig.  27. 


0.85ya 


0.9// 


0.95a 


being  2.5,  5,  10,  and  20  mm.  Figs.  26  to  29,  inclusive,  represent  these 
results,  the  abscissae  being  wave-lengths  and  the  percentage  transmissions 
being  given  as  ordinates.  It  is  seen  that  the  absorption  bands  in  the  nitrate 
solutions,  as  with  those  of  the  chloride  discussed  above,  show  three  minima 
at  X7300,  X7950,  and  X8750.    The  nitrate  bands  are  not  as  intense  as  those 


AS   MEASURED   BY   MEANS   OF   THE   RADIOMICROMETEK. 


73 


of  the  concentrated  solutions  of  the  chloride  given  in  figs.  18  to  21.  This 
is  what  we  should  expect,  since  the  concentrations  of  the  nitrate  solutions 
are  not  so  great.  However,  two  of  the  absorption  bands  reach  zero  trans- 
mission. A  comparative  study  of  any  of  these  bands  in  the  succeeding 
curves  shows  that,  just  as  was  found  with  the  chloride  bands,  they  become 


100 


75 


50 


25 


Neodymium  Nitrate 

Cell  Depth   10  mm. 
Concentration  0.525  N 


0.65a 


0.7// 


0.75m  O.Bju- 

Fig.  28. 


q.85/1 


0.9a 


0,95a 


100 


75 


50 


25 


Neodymium  Nrtrate 

Cell  Depth  20  mm. 
Concentration  0.262  N 


0.65a 


0.75a  0.8a 

Fig.  29. 


0.95a 


more  intense  with  dilution.  The  decided  decrease  in  the  transmission  in  the 
regions  of  the  spectrum  beyond  Ifx  is  undoubtedly  due  to  the  increasing 
absorption  of  water  as  the  solution  becomes  more  dilute.  The  other  slight 
deviations  from  Beer's  law  are  not  greater  than  could  be  accounted  for  by 
the  corrections  mentioned  under  the  discussion  of  the  chloride  curves. 


74 


ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 
NEODYMIUM  ACETATE. 


Table  15  gives  the  results  obtained  for  solutions  of  neodymium  acetate  in 
water.  The  concentrations,  reading  from  left  to  right,  were  0.G17,  0.308, 
0.154,  and  0.077  normal,  respectively,  the  corresponding  depths  of  absorbing 


100 


2  75 

in      '-' 


{.■5 
< 

I  50 


25 


Neodyrnium  Acetate 

Cell  Depth  2.5  mm. 
Concentration  0.6I7N 


0.65/^ 


100 


0.7/^ 


0.75// 


Fig. 


O.Su 

30. 


0.85m 


0.9m 


0.95m 


75 


^50 


25 


Neodymium  Acetate 

Cell  Depth    5  mm. 
Concentration  0.308 N 


0.65m 


0.7m 


0.75//  ^«/ 

Fig.  31. 


0.85m 


0.9m 


0.95m 


layer  being  2.5,  5,  10,  and  20  mm.,  respectively.  The  results  in  this  table 
are  plotted  in  figs.  30  to  33,  inclusive.  The  percentage  transmission  and 
wave-lengths  are  represented,  respectively,  by  the  ordinates  and  al)r<^is?[e  of 
the  curves. 


AS   MEASURED    BY   MEANS   OF   THE   KADIOMICROMETER. 


75 


The  minima  of  transmission  fall  at  approximately  the  same  positions  as 
with  the  chloride  and  nitrate  solutions  discussed  above,  i.  e.,  at  X7300,  X7950, 
and  X8750.  As  indicated  by  the  photographic  method,  the  solutions  of  neo- 
dymium  acetate  have  greater  absorbing  powers  for  a  given  concentration 
than  either  the  chloride  or  nitrate. 


100 


75 


»-   50 

lij 

UJ 
UJ 

CL 


25 


Neodymium  Acetate 

Cell  Depth    10  mm. 
Concentration  0.154 N 


0.65/U 


0.7a 


0.75/^  0.8/Z 

Fig.  32. 


0.85// 


0.9m 


0.95/1 


100 


75 


50 


25 


Neodymium  Acetate 

Cell  Depth  20mm. 
Concentration  0.077  N 


0.65// 


0.7// 


0.75// 


0.8// 

Fig.  33. 


0.85// 


0.9// 


0.95// 


The  small  band  near  X7000  appears  slightly  more  intense  with  the  acetate 
than  with  equal  concentrations  of  the  other  salts;  and  the  more  intense  bands 
X7300,  X7950,  and  X8750  show  the  same  general  tendency  to  have  their 
minima  lowered  with  increasing  dilution.  This  set  of  curves  shows  in  a 
marked  way  the  rapid  increase  in  the  absorption  near  X9500,  which  is  due  to 
the  water  present  in  the  solution  and  illustrates  the  difficulty  that  is  met 


76 


ABSOKPTION   SPECTRA.   OF  A  NUMBER   OF   SALTS 


with  when  working  with  aqueous  sohitions  at  greater  wave-lengths  than  Ifx. 
Even  over  the  range  of  wave-lengths  at  which  we  have  worked,  it  is  seen 
that  the  absorption  due  to  water  is  a  disturbing  factor. 


Table  15 

. — Trans7nissions  of  Neodymium  Acetate  Solutions. 

X 

D.=2.5mm.;D.=5  mm. 

D.  =  10mm. 

D.=20mm.  ;   a 

D.=2.5mm.  D.=5  mm. 

D.  =  10mm. 

D.=20mm. 

C.=0.617N.jC.=0.308N. 

C.=0.154N. 

C.=0.077N.| 

A 

C.=0.617N.1C.=0.308N. 

i 

C.=0.154N. 

C.=0.077N. 

686 

98      97 

94 

94 

800 

27  ■:        28 

27 

28 

693 

92 

91 

93 

95 

805 

57      46 

44 

46 

698 

97 

97 

96 

93 

809 

69   !   63 

62 

64 

704 

98 

100 

92 

93 

814 

74  \        76 

73 

75 

708 

96   i   98 

92 

96   i  819 

90   i   83 

83 

82 

712 

100   ;   96 

94 

96 

825 

92   1   89 

86 

87 

716 

100   i   94 

90 

94 

830 

92      93 

92 

91 

720 

94      90 

90 

94 

834 

98   1   98 

92 

91 

723 

87   ,   87 

79 

76 

839 

96 

97 

94 

90 

726 

69 

64 

60 

57 

845 

97 

97 

90 

88 

730 

44 

36 

32 

30 

850 

95 

96 

90 

87 

733 

22 

22 

18 

21 

856 

95 

94 

84 

83 

737 

24 

17 

14 

15 

861 

91 

83 

77 

71 

741 

30 

23 

22 

25 

867 

72 

69 

65 

67 

746 

41 

39 

34 

35 

872 

62 

60 

56 

53 

751 

52 

55 

52 

55 

877 

66 

67 

58 

57 

755 

71 

71 

65 

67 

882 

71 

72 

66 

66 

759 

81 

84 

81 

82 

888 

83   i   82 

76 

75 

763 

90 

91 

87 

88 

894 

87 

84 

81 

78 

767 

93 

93 

89 

87 

900 

91 

88 

84 

79 

770 

94 

90 

87 

89 

906 

92 

90 

87 

83 

774 

89 

88 

83 

84 

912 

94   ;   90 

90 

82 

779 

72 

74 

74 

75 

917 

96   1   93 

89 

85 

783 

62 

63 

59 

55 

923 

97   I   96 

94 

86 

787 

45 

40 

37 

32 

933 

99 

95 

91 

83 

791 

24 

21 

17 

18 

938 

99 

95 

92 

77 

796 

22 

17 

17 

14 

PRASEODYMIUM  CHLORIDE. 

Solutions  of  praseodymium  salts  are  not  of  great  interest  from  our  stand- 
point, in  those  regions  beyond  the  visible  part  of  the  spectrum.  It  was 
found  that  such  solutions  were  transparent  in  the  infra-red  end  of  the  spec- 
trum as  far  as  1.5,  except  two  very  weak  bands  which  fall  just  in  the  midst  of 
the  intense  water-bands.  Since,  at  this  point,  a  very  thin  layer  of  water  is 
almost  completely  opaque,  it  is  evident  that  it  would  be  impossible  to  study 
aqueous  solutions  in  this  region,  especially  dilute  solutions. 

As  shown  by  the  photographic  plate,  praseodymium  salts  possess  two 
groups  of  bands  in  the  visible  spectrum,  one  in  the  green  near  X4600  and 
another  near  X5900.  Since  the  amount  of  energy  at  the  former  wave-length 
is  so  very  small,  the  width  of  slits  necessary  to  be  used  was  too  large  to  give 
satisfactory  results.  Such  bands  could,  of  course,  be  detected,  but  the 
deflections  of  the  instrument  at  this  part  of  the  spectrum  are  very  small, 
and,  hence,  relatively  large  errors  would  occur  in  making  the  readings. 

For  these  reasons  we  have  confined  our  attention  to  a  careful  study  of  the 
one  band  which  has  its  center  near  X5900.  Table  16  gives  the  observed 
transmissions  for  the  four  dilutions  of  solutions  of  praseodymium  chloride. 


AS   MEASURED   BY   MEANS   OF   THE   RADIOMICROMETER. 


77 


In  column  1  there  is  given  X  taken  at  such  intervals  as  the  graduated  head  of 
the  spectroscope  would  permit.  Reading  from  left  to  right,  the  respective 
concentrations  were  1.377,  0.688,  0.344,  and  0.177  normal,  the  corresponding 
depths  of  absorbing  layer  being  2.5,  5,  10,  and  20  mm.,  respectively. 

Table  16. — Percentage  Transmissions  of  Praseodymium  Chloride  Solutions. 


X 

D.  =  2.5mm.iD.=5  mm. 

D.  =  10mm. 

D.=20  mm. 

X 

D.=2.5mm. 

D.=5  mm. 

D.=10  mm. 

D.=20  mm. 

C.  =  1.377N. 

C.=0.688N. 

C.=0.344N. 

C.=0.177N. 

C.-1.377N. 

C.=.0.688N. 

C.=0.344N. 

C.=0.177N. 

506 

100 

100 

100 

100 

587 

60 

59 

55 

56 

618 

100 

98 

100 

100 

589 

45 

46 

40 

40 

530 

100 

98 

100 

100 

592 

35 

35 

32 

33 

544 

99 

99 

99 

98 

595 

35 

34 

34 

33 

556 

99 

99 

99 

98 

597 

43 

43 

42 

42 

563 

100 

98 

100 

98 

600 

56 

56 

56 

56 

565 

99 

99 

98 

97 

602 

69 

69 

68 

69 

567 

97 

100 

98 

97 

605 

81 

82 

81 

81 

572 

97 

99 

95 

97 

607 

90 

92 

88 

88 

577 

96 

98 

93 

95 

611 

93 

94 

93 

94 

579 

91 

92 

90 

92 

614 

97 

99 

95 

97 

583 

85 

86 

84 

80 

629 

98 

100 

97 

98 

585 

77 

76 

70 

72 

In  terms  of  Beer's  law,  the  curves  representing  these  tables  should  be  iden- 
tical. Such  curves  are  represented  by  fig.  34.  Beginning  with  the  curve 
nearest  the  left  and  proceeding  towards  the  right,  the  succeeding  curves  rep- 
resent the  four  dilutions  of  praseodymium  chloride  as  given  in  the  preceding 
paragraph. 


100- 


75- 


50 


25 


Cell  Depth  2.5  mm. 
Concentration  1.37  7  N 


Cell  Depth  5  mm. 
Concenrration0.688N 


Cell  Depth  10  mm. 
Concentration  0344  N 


Cell  Depth  20  mm. 
Concentration  0.177  N 


0.55m 


Q.6/U       0.55/z 


0.6/z        0.55/z 

Fig.  34, 


0..6/Z        0.55ju 


0.6/jL 


The  curve  representing  the  most  concentrated  solution  is  nearest  the  left 
of  the  figure.  It  is  seen  that  these  curves  are  identical  to  within  the  limits  of 
experimental  error;  the  slight  increase  in  the  absorption  with  dilution  is  to 
be  attributed  to  the  slit-width  correction.  The  slit-width  was  in  every  case 
0.4  nrni.     Water  has  no  absorption  in  this  region. 


78 


ABSORPTION   SPECTRA    OF   A   NUMBER   OF   SALTS 


The  results  recorded  in  these  curves  are  in  agreement  with  previous  photo- 
graphic results.  The  minimum  in  transmission,  which  in  each  case  is  about 
30  per  cent,  occurs  near  X5900.  The  total  deviation  from  Beer's  law  over 
the  dilution  studied  is  not  over  3  per  cent,  which  is  well  within  the  experi- 
mental error  in  this  portion  of  the  spectrum. 

PRASEODYMIUM  NITRATE. 
Corresponding  results  for  solutions  of  praseodymium  nitrate  are  given  in 
table  17.  The  concentrations  of  the  solutions,  beginning  on  the  left  and 
reading  towards  the  right,  were  1.282,  0.641,0.320,  and  0.160  normal,  respec- 
tively, the  corresponding  depths  of  absorbing  layer  being  2.5,  5,  10,  and 
20  mm.,  respectively. 

Table  17. — Percentage  Transmission  of  Praseodymium  Nitrate  Solutions. 


X 

D.=2.5mm. 

D.=5  mm. 

D.=lOmm. 

D.=20mm. 

X 

D.=2.5mm. 

D.=5mm. 

D.  =  10mm. 

D.=20mm. 

C.=1.282. 

C.  =0.641. 

C.  =0.320. 

C.  =0.160. 

C.  =  1.282. 

C.  =0.641. 

C. =0.320. 

C.  =0.160. 

506 

100 

100 

99 

100 

587 

62 

60 

60 

58 

518 

98 

98 

100 

100 

589 

48 

46 

46 

45 

530 

100 

100 

99 

99 

592 

40 

40 

38 

36 

544 

99 

98 

100 

100 

595 

37 

36 

36 

36 

556 

99 

98 

100 

98 

597 

44 

44 

44 

42 

563 

99 

99 

98 

97 

600 

56 

56 

56 

54 

565 

96 

100 

98 

96 

602 

68 

67 

68 

67 

567 

98 

100 

98 

97 

605 

79 

82 

79 

80 

572 

95 

97 

96 

95 

i  607 

87 

89 

89 

88 

577 

93 

92 

94 

93 

i  611 

91 

92 

94 

92 

579 

90 

92 

90 

91 

1  614 

95 

95 

97 

96 

583 

83 

86 

85 

83 

!  629 

98 

98 

99 

98 

585 

73 

74 

72 

72 

1 

Cell  Depth  2.5mm. 
Concentration  1.282  N 


Cell  Depth  5mm. 
Concentration  0.641  N 


Cell  Depth  10  mm. 
Concentration  0.320N 


Cell  Depth  20mm. 
Concentration  0  I60N 


0.55/^ 


0.6m        0.55m 


0.6m        0.55// 

Fig.  35. 


0.6m        0.55m 


0.6m 


The  results  are  plotted  in  fig.  35.  Reading  from  left  to  right,  there  is 
shown  the  effect  of  increased  dilution  on  solutions  of  praseodymium  nitrate 
and  such  concentrations  and  depths  of  absorbing  layers  as  were  mentioned 
above,  the  most  concentrated  solution  being  nearest  the  left  of  the  figure. 


AS   MEASURED   BY   MEANS   OP   THE   RADlOMlCROMETER. 


79 


As  with  the  curves  representing  the  sohitions  of  praseodymium  chloride, 
these  curves  show  that  Beer's  law  holds  quantitatively  for  solutions  of  the 
nitrate.  Neither  the  position  nor  the  intensity  of  the  band  is  altered  more 
than  the  limits  of  error  of  our  work  for  the  range  of  dilution  studied.  It 
may  be  recalled  that  this  is  in  exact  agreement  with  the  photographic  results 
recorded  elsewhere  in  this  monograph. 

SALTS  OF  NICKEL. 

Table  18  gives  the  percentage  transmission  of  the  nickel  salts  studied. 
Beginning  at  the  left  of  the  table,  column  1  gives  X,  taken  at  such  intervals 
as  the  solutions  required,  and  reading  towards  the  right  are  the  results  for 
the  following  salts:  nickel  chloride,  depth  of  cell  3  mm.,  concentration  2.74 
normal;  nickel  nitrate,  depth  of  cell  5  mm.,  concentration  1.68  normal ;  nickel 
sulphate,  depth  of  cell  5  mm.,  concentration  1.108  normal,  respectively. 

Curves  representing  these  results  are  given  in  figs.  36,  37,  and  38. 

Table  18. — Percentage  Transmissions  of  Solutions  of  Nickel  Salts. 


Nickel 

Nickel 

Nickel 

Nickel 

Nickel 

Nickel 

X 

chloride. 

nitrate. 

sulphate. 

X 

chloride. 

nitrate. 

sulphate. 

D.=3  mm. 

D.=5mm. 

D.=5mm. 

D.=3  mm. 

D.=5  mm. 

D.=5  mm. 

C.=2.74  N. 

C.  =  l.68N. 

C.  =  1.108  N. 

C.=2.74  N. 

C.  =  1.68  N. 

C.  =  1.108N. 

544 

71 

69 

81    1 

796 

16 

20 

33 

556 

64 

69 

79   ; 

805 

18 

24 

40 

563 

62 

65 

76   1 

814 

22 

31 

44 

565 

60 

61 

73 

825 

28 

36 

49 

577 

56 

57 

68   I 

&34 

32 

40 

55 

583 

52 

50 

66   ! 

845 

36 

46 

58 

587 

47 

46 

61    i 

856 

41 

48 

64 

592 

40 

40 

60   I 

867 

46 

51 

66 

597 

34 

33 

40 

877 

49 

52 

68 

602 

26 

28 

43 

888 

51 

56 

69 

607 

22 

22 

37 

900 

53 

56 

68 

614 

17 

17 

33 

912 

54 

56 

67 

618 

12 

12 

26 

923 

52 

51 

64 

625 

10 

9 

23 

933 

50 

48 

62 

632 

8 

6 

18 

938 

47 

44 

59 

638 

5 

6 

15 

965 

42 

38 

52 

643 

3 

4 

12 

966 

36 

-  30 

44 

650 

0 

4 

12 

978 

30 

24 

38 

662 

0 

3 

10 

990 

26 

20 

33 

676 

0 

2 

10 

1002 

22 

16 

28 

693 

0 

9 

11 

1012 

20 

14 

24 

755 

7 

8 

10   1 

1023 

15 

9 

22 

770 

7 

8 

18   1 

1035 

13 

8 

20 

779 

11 

13 

20   ! 

1047 

9 

4 

17 

787 

14 

17 

30   1 

1060 

6 

4 

14 

Nickel  Chloride. 

Fig.  36,  the  curve  for  nickel  chloride,  shows  an  increasing  absorption  from 
70  per  cent  transmission  at  X5200  to  complete  absorption  at  X6300.  From 
this  point  there  is  complete  absorption  to  the  regions  X7200,  and  then  a 
gradual  increase  in  transmission,  reaching  a  maximum  of  53  per  cent  near 
X9000,  then  decreasing  again  to  zero  transmission  at  XIOOOO. 

As  has  been  shown  photographically,  the  visible  spectrum  of  salts  of 
nickel  consists  of  intense  broad  absorption  bands  both  in  blue  and  red, 


80 


ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 


showing  a  single  region  of  transmission,  extending  to  about  X6500  in  the  red. 
It  is  interesting  to  note  that  by  means  of  the  radiomicrometer  we  are  able  to 
study  another  region  of  transmission  which  reaches  a  maximum  near  X9000. 
From  this  point  the  absorption  rapidly  increases  until  the  region  of  water 
absorption  is  reached.  It  was  not  possible  to  determine  whether  the  solu- 
tion again  became  transparent  beyond  this  point,  on  account  of  the  water 
absorption. 


50 


25 


0.5;u 


Nickel  Chloride 

Cell  Depth  3mm 
Concentration  2  74N 


0.6a 


•01m 


08// 

Fig.  36. 


118 


Nickel  Nitrate. 

Fig.  37,  w^hich  represents  the  second  column  of  table  18,  gives  the  trans- 
mission, as  observed,  for  a  solution  of  nickel  nitrate,  concentration  1.68  nor- 
mal and  5  mm.  absorbing  layer.  This  j&gure  is  almost  exactly  analogous  to 
the  curve  for  nickel  chloride,  just  discussed  in  the  preceding  paragraph,  and 
shows  maxima  at  X5400  and  X9000.  There  is  complete  absorption  in  the 
region  X7000  and  beyond  l.ljit.  Figs.  36  and  37  represent  approximately 
equal  amounts  of  the  chloride  and  nitrate  respectively,  and  are  very  similar. 


75 


50 


0.5/z 


NicUel  Nitrate 

Cell  Depth  5  mm. 
Concentration  1.68  N 


Q&u 


0.7m 


0.8a  0.9m 

Fig.  37. 


Nickel  Sulphate. 

The  curve  representing  the  last  column  of  this  table  is  given  in  fig.  38. 
The  concentration  of  nickel  sulphate  was  1.108  normal,  and  the  depth  of 
absorbing  layer  was  5  mm.  It  will  be  seen  that  this  solution  is  slightly  more 
dilute  than  the  other  two  solutions  of  nickel  salts  studied;  and  that  in  no 


AS   MEASURED   BY   MEANS   OF   THE   RADIOMICROMETER. 


81 


region  does  the  curve  representing  the  transmission  reach  complete  absorp- 
tion. Maxima  in  transmission  appear  near  X5400  and  X9000  and  minima 
of  about  8  per  cent  at  X6900  and  XI 1000.  Readings  were  not  made  at 
greater  wave-lengths  with  any  of  the  nickel  salts,  on  accoxmt  of  the  intense 
water  absorption  slightly  beyond  this  region. 

No  striking  or  characteristic  difference  is  noted  in  the  absorption  as  shown 
by  the  three  curves  representing  the  three  salts  of  nickel  studied.  Each  of 
them  shows  maxima  and  minima  in  about  the  same  region  of  the  spectrum. 


S50 
< 


0.5a 


0.6m 


0.7// 


0.8a 

Fig.  38. 


OSju 


Um 


SALTS  OF  COBALT. 

The  photographic  plate  shows  that  in  the  visible  region  salts  of  cobalt 
have  a  strong  ultra-violet  absorption,  a  band  in  the  orange  near  X5000,  and 
increasing  transmission  from  the  center  of  this  band  toward  the  longer 
wave-lengths.  It  was  interesting  to  see  whether  solutions  of  cobalt  salts 
were  completely  transparent  beyond  the  limit  of  sensibility  of  the  photo- 
graphic plate,  or  if  such  solutions  again  showed  absorption  bands  in  the 
infra-red.  It  was  also  of  interest  to  know  whether  equal  concentrations  of 
the  different  salts  showed  the  same  or  different  absorption  bands.  With  this 
idea  in  view,  the  solutions  of  five  salts  of  cobalt  were  studied.  In  each  case 
the  concentration  was  0.347  normal,  and  the  depth  of  absorbing  layer  10  ram. 

The  results  are  given  in  table  19.  Beginning  at  the  left  are  given,  in  their 
respective  order,  the  observed  transmissions  for  10  mm.  of  solution  of  the 
following  salts:  cobalt  chloride,  cobalt  bromide,  cobalt  nitrate,  cobalt  sul- 
phate, and  cobalt  acetate. 

The  results  of  table  19  are  plotted  as  transmission  curves  in  figs.  39  to  43, 
inclusive.  A  study  of  these  curves  shows  that  they  are  very  similar,  all 
having  maxima  of  transmission  at  the  following  points :  X5950,  X7800,  X91 00, 
and  X10,600.  In  general,  for  all  the  salts  of  cobalt  studied,  the  transmission 
curves  rise  rapidly  from  X5000  to  X5900,  where  the  transmission  reaches 
about  65  per  cent.     The  curves  show  a  broad  but  slight  absorption  over  the 


82 


ABSORPTION   SPECTRA   OF   A   NtJMBER   OF   SALTS 


region  near  X6500,  and  reach  a  maximum  transmission  over  tlie  region  near 
X7000  to  X8000.  The  curves  then  descend,  showing  a  series  of  small  absorp- 
tion regions  near  X8400,  X8900,  and  X9800.  The  last  of  these  absorption 
bands  shows  a  fairly  sharp  edge  toward  the  shorter  wave-lengths.  Beyond 
X10,500  the  absorption  increases  rapidly  until  the  region  is  reached  where 
water  is  practically  opaque. 

Table  19. — Transinissions  of  Cobalt  Solutions. 


Cobalt  chloride. 

Cobalt  bromide. 

Cobalt  nitrate. 

Cobalt  sulphate. 

Cobalt  acetate. 

X 

D.  =  10mm. 

D.  =  10mm. 

D.  =  10mm. 

D.  =  10mm. 

D.  =  10  mm. 

C.  =0.347  N. 

C.  =0.347  N. 

C. =0.347  N. 

C.  =0.347  N. 

C.  =0.347  N. 

544 

12 

1     21 

20 

14 

15 

556 

19 

33 

31 

26 

31 

572 

39 

50 

35 

48 

53 

583 

48 

61 

64 

57 

59 

589 

59 

68 

68 

62 

69 

592 

63 

74 

73 

67 

72 

605 

68 

74 

77 

67 

72 

614 

70 

74 

77 

68 

74 

622 

72 

74 

79 

68 

75 

632 

72 

75 

84 

69 

76 

640 

72 

75 

76 

70 

77 

650 

73 

76 

79 

71 

76 

660 

73 

78 

81 

72 

80 

667 

74 

78 

82 

74 

83 

678 

79 

82 

85 

77 

84 

686 

80 

84 

88 

79 

85 

693 

81 

85 

90 

88 

88 

708 

84 

87 

91 

81 

89 

720 

84 

89 

91 

82 

92 

730 

85 

87 

92 

83 

89 

741 

88 

88 

88 

82 

91 

755 

87 

89 

89 

83 

91 

767 

87 

i     92 

89 

82 

92 

779 

88 

89 

89 

84 

90 

791 

90 

89 

89 

83 

90 

805 

91 

89 

87 

84 

89 

819 

88 

88 

84 

83 

90 

834 

87 

87 

83 

83 

86 

850 

88 

86 

76 

80 

86 

867 

88 

95 

95 

80 

85 

882 

84 

84 

82 

79 

83 

900 

84 

81 

77 

76 

84 

917 

83 

1     82 

79 

77 

82 

933 

78 

77 

79 

73 

78 

949 

70 

69 

70 

66 

70 

966 

68 

55 

56 

54 

56 

984 

51 

i     49 

50 

47 

52 

1,002 

49 

48 

50 

^ 

49 

1,018 

51 

50 

50 

47 

52 

1,035 

51 

1     50 

53 

50 

52 

1,053 

48 

1     50 

56 

49 

52 

1,072 

46 

:    46 

50 

44 

48 

1,091 

41 

41 

46 

40 

41 

1,109 

34 

35 

39 

34 

36 

1,123 

29 

31 

35 

30 

32 

1,136 

24 

25 

23 

24 

25 

1,147 

17 

21 

14 

16 

19 

1,162 

12 

12 

13 

11 

12 

1,174 

9 

10 

11 

10 

10 

1,187 

9 

9 

10 

9 

9 

1,202 

8 

8 

^ 

8 

10 

AS   MEASURIiD   BY   MEANS   OF   THE   RADlOMlCROMETER. 


83 


100 


Cobalt  Chloride 

Cell  Depth  lOmm. 
Concontra-tion  0.347 N 


O.JM 


0.6m 


0.7^ 


O.SyU  0.9/z 

Fig.  39. 


1.1/. 


1.18 


100 


50 


Cobalt  Bromide 

Cell  Depth  lOmm. 
Concentration  0.347  N 


0.5m 


0.6m 


0.7m 


0.8//  0.9/. 

Fig.  40. 


1./Z 


l.lA 


1.18 


100 


50 


25 


Cell  Depth  lOmm. 
Concentration  0.347  N 


0.5m 


0.6// 


07/< 


0.8// 

Fig.  41. 


0.9m 


1.// 


Um 


84 


ABSORPTION   SPECTRA   OF  A   NUMBER   OF   SALTS 


Fig.  42,  representing  the  curve  of  transmission  of  cobalt  sulphate,  appears 
slightly  different  from  the  curves  of  the  other  salts  of  cobalt,  having  better- 
defined  bands  in  the  region  of  X8000. 


75 


50 


25 


Ce If  Depth  10 mm. 
Concentration  0.347  N 


03a 


0.6// 


0.7// 


0.8//  0.9// 

Fig.  42. 


'•/" 


1.18 


too 


75 


50 


25 


5a 


0.8a 

Fig.  43. 


CHAPTER  VII. 

GENERAL  SUMMARY  OF  RESULTS. 

The  work  on  the  effect  of  temperature  on  the  absorption  spectra  of  solutions 
was  extended  to  aqueous  solutions,  the  range  in  temperature  being  from 
ordinary  temperatures  up  to  about  200°.  For  this  purpose  a  special  form  of 
apparatus  was  constructed,  made  of  brass  and  lined  on  the  inside  with  gold. 
This  was  for  the  purpose  of  preventing  the  hot  vapor  under  high  pressure 
from  coming  in  contact  with  any  metal  except  gold. 

With  this  apparatus  the  absorption  spectra  of  aqueous  solutions  could  be 
studied  up  to  200°,  just  as  well  as  the  spectra  of  nonaqueous  solutions  in  the 
apparatus  used  by  Jones  and  Strong,  and  described  in  Publication  of  the 
Carnegie  Institution  of  Washington  No.  160. 

With  neodymium  chloride  the  following  bands  remain  unchanged  by 
temperature  over  the  range  from  20°  to  200°:  X3800,  X4025,  X4200,  X4325, 
X4440,  X4600,  X4690,  X4750,  X4820.  The  double  band  X5050  to  X5270 
changes  very  slightly.  The  bands  X4275  and  X5800  show  marked  changes, 
the  red  edge  widening  and  becoming  more  diffuse.  The  X5800  band  widens 
as  much  as  50  a.u.  toward  the  red,  the  violet  edge  remaining  sharp. 

With  the  neodymium  bromide,  as  with  the  chloride,  only  X4275  and  X5800 
show  any  marked  changes  with  rise  in  temperature.  The  band  X5800  widens 
for  the  bromide  60  a.u.  from  20°  to  190°. 

The  X4275  band,  for  neodymium  nitrate,  shows  a  marked  change,  widen- 
ing towards  the  red.  The  X4425  band  widens  about  15  a.u.  from  15°  to  165°. 
The  bands  X5125  and  X5800  show  marked  changes  towards  the  red.  The 
change  was  greatest  in  the  most  concentrated  solutions,  although  the  total 
number  of  absorbers  in  the  path  of  light  was  kept  constant. 

The  bands  X4275  and  X5800  for  neodymium  acetate  show  marked  changes 
on  the  red  side,  the  latter  widening  as  much  as  80  a.u.  The  acetate  bands, 
for  a  given  concentration  of  salt,  are  the  most  intense  of  all  the  neodymium 
bands.  When  the  solution  was  cooled  down,  the  absorption  spectra  went 
through  exactly  the  reverse  changes  as  when  the  temperature  was  raised. 
Since  the  acetate  band,  X5800,  is  more  intense  for  the  same  concentration 
than  for  neodymium  chloride  or  nitrate,  and  since  the  acetate  is  less  dis- 
sociated than  the  neodymium  salts  of  the  strong  acids,  it  appears  probable 
that  this  band  is  in  some  way  connected  with  the  molecules. 

The  sulphate  of  neodymium  shows  the  same  temperature  effect  as  the 
other  salts  of  this  element. 

The  effect  of  temperature  on  the  absorption  spectra  of  cobalt  chloride  is 
very  slight. 

There  is  a  slight  widening  of  the  band  of  praseodymium  chloride  whose 
center  is  near  X4825.     The  X5900  band  undergoes  slight  change  with  tem- 

85 


86  ABSORPTION   SPECTliA   OF   SOLUTIONS. 

perature,  but  from  20°  to  160°  it  changes  less  than  25  a.u.  There  is  no 
appreciable  change  with  temperature  of  the  l^ands  whose  centers  are  near 
X4425,  X4650,  and  X4820. 

The  above-described  temperature  changes  take  j^lace  only  in  concentrated 
solutions.  In  very  concentrated  solutions  all  of  the  praseodymium  bands 
show  a  slight  widening  with  rise  in  temperature.  We  shall  see  that  increase 
in  dilution  affects  the  bands  of  praseodymium  salts  only  when  the  solutions 
are  fairly  concentrated.  Thus,  rise  in  temperature  and  increase  in  concen- 
tration produce  the  same  effect  on  the  absorption  spectra  of  solutions  of 
praseodymium  nitrate. 

The  effect  of  rise  in  temperature  on  the  absorption  spectra  of  solutions  of 
uranyl  nitrate  is  a  general  widening  of  the  bands,  with  a  slight  shift  of  the 
center  towards  the  red.  The  general  absorption  ending  near  X3500  moves 
rapidly  towards  the  red  with  rise  in  temperature.  All  of  the  eleven  bands 
between  X3500  and  X4600  become  more  diffuse  and  broader  with  rise  in 
temperature,  the  X4180  band  being  most  affected.  The  red  edge  of  this 
band  shifts  as  much  as  25  a.u.  from  40°  to  120°. 

The  uranyl  sulphate  bands  X4175  and  X4325  have  their  centers  shifted 
towards  the  red  about  25  a.u.  for  a  temperature  range  of  from  20°  to  185°. 
The  band  X4750  remains  unchanged,  while  the  red  edges  of  X4325  and  X4550 
shade  rapidly  towards  the  red.  All  bands  below  X4500  become  very  diffuse 
as  the  temperature  is  raised,  and  at  the  highest  temperatures  are  a  single, 
broad,  hazy  absorption  band  extending  from  X4000  to  X4400. 

The  most  marked  widening  is  in  the  uranyl  sulphate  bands  X4100,  X4200, 
and  X4350,  the  center  of  each  of  these  bands  being  slightly  shifted  towards 
the  red.  The  broad,  hazy  bands  X5100,  X5600,  and  X6200  are  not  appreci- 
ably affected  by  changes  in  temperature. 

None  of  the  uranyl  acetate  bands  seems  to  undergo  change  with  dilution; 
all  of  the  nine  bands  on  the  plate  undergo  change  with  rise  in  temperature, 
becoming  more  diffuse. 

While  some  of  the  absorption  bands  of  solutions  are  practically  unaffected 
by  temperature,  many  of  them  widen  as  the  temperature  is  raised.  The 
effect  of  rise  in  temperature  is  not  to  produce  a  symmetrical  widening  of 
the  bands,  but  most  of  the  widening  is  towards  the  red.  The  violet  edge  of 
the  band  usually  remains  pretty  sharp.  The  red  edge  widens  out,  becoming 
more  hazy  and  diffuse. 

The  effect  of  dilution  on  the  absorption  of  light  by  solutions  was  early 
studied  by  Ostwald  and  others,  especially  in  connection  with  the  theory  of 
electrolytic  dissociation.  It  was  known  that  both  molecules  and  ions  in 
solution  absorb  light,  and  the  question  was  whether  they  have  the  same  or 
different  absorption.  It  was  not  possible  to  answer  this  question  satis- 
factorily by  means  of  the  prism  spectroscope.  It  has  been  possible  to  solve 
this  problem  by  means  of  the  grating. 

Jones  and  Anderson  had  shown  that  if  molecules  and  ions  absorb  differ- 
ently, the  difference  is  slight.     We  therefore  worked  over  a  wide  range  in 


GENERAL   SUMMARY   OF   RESULTS.  87 

dilution,  comparing  the  absorption  of  a  concentrated  solution  with  one  500 
times  as  dilute. 

The  neodymium  chloride  bands  X3400,  X3450  to  X3600  are  not  affected  by 
change  in  dilution.  The  sharp  band  X4275  is  more  intense  in  the  most  con- 
centrated solution.  The  bands  near  X5100,  X5200,  and  X5800  are  markedly 
affected  by  dilution,  the  former  two  appearing  as  distinct  bands  in  the  most 
dilute  solution.  The  broadening  of  these  bands  with  concentration  is  fairly 
uniform,  both  towards  the  red  and  the  violet  ends  of  the  spectrum.  The 
intense  band  from  X5690  to  X5850  is  greatly  affected  by  concentration, 
widening  almost  entirely  towards  the  red.  This  widening  is  about  50  a.u. 
When  a  more  dilute  solution  was  used  to  start  with,  the  broad  band  X5700  to 
X5825  is  the  only  one  which  widens  with  increase  in  concentration,  the  widen- 
ing being  about  25  a.u.  When  a  still  more  dilute  solution  is  used  as  the 
starting-point,  there  is  no  appreciable  change  in  any  of  the  bands  with 
increase  in  concentration. 

With  neodymium  bromide,  with  increase  in  concentration  there  is  a 
slight  increase  in  the  intensity  of  X4275.  The  bands  X5090,  X5120,  andX5210 
narrow  uniformly  with  dilution.  The  greatest  change  is  in  band  X5750, 
which  widens  towards  the  red  as  much  as  30  a.u.,  the  violet  edge  remaining 
practically  unchanged.  When  half  the  concentrations  were  used,  the  only 
band  affected  by  dilution  is  the  one  near  X4800,  which  widens  with  the  con- 
centration as  much  as  20  a.u.  When  all  of  the  dilutions  were  again  doubled, 
there  was  practically  no  difference  between  the  absorption  spectra  of  the 
various  dilutions. 

The  effect  of  dilution  on  the  absorption  spectra  of  neodymium  nitrate  is 
probably  greater  than  on  any  other  neodymium  salt.  Then  bands  X5090  and 
X5125,  in  the  most  concentrated  solution,  have  so  broadened  as  to  become 
one  band.  The  band  X5220  widens  uniformly  towards  both  the  red  and 
violet  with  increase  in  concentration  probably  as  much  as  70  a.u.  Starting 
with  a  different  concentration,  the  X5750  band  widens  as  much  as  40  a.u.  as 
the  concentration  is  changed.  When  the  original  solution  was  still  more 
dilute,  only  the  X5750  band  changed  appreciably. 

The  band  X5750  of  neodymium  sulphate  widens  with  concentration  as 
much  as  25  A.u.  It  remains  unchanged  if  the  initial  solution  of  neodymium 
sulphate  is  more  dilute. 

A  number  of  the  bands  of  neodymium  acetate  change  with  the  dilution. 
The  X5210  band  narrows  about  10  a.u.  with  the  first  change  in  dilution,  and 
then  remains  unchanged  with  further  increase  in  dilution.  The  broad  band 
X5750  changes  about  55  a.u.  with  the  change  in  dilution  studied.  When 
only  half  the  initial  concentrations  were  used,  only  the  bands  X5220  and 
X5750  underwent  change.  With  neodymium  acetate  further  increase  in 
dilution  produced  still  further  narrowing  of  the  absorption  bands.  When 
one-fourth  the  initial  concentration  was  used,  the  band  X5750  underwent 
change,  narrowing  about  20  a.u.  This  is  the  only  salt  of  neodymium  in 
which  a  change  in  a  band  was  noted  at  such  a  high  dilution. 


88  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

The  X4675  band  of  praseodymium  chloride  narrows  towards  the  violet 
about  20  A.u.  with  increase  in  dilution,  while  the  broad  band  X5900,  under 
the  same  conditions,  shows  a  narrowing  of  about  25  a.u.  When  more  dilute 
solutions  are  employed,  none  of  the  bands  shows  any  change  with  dilution. 
The  changes  in  the  two  bands  X4675  and  X5900  with  dilution  are  much  less 
than  with  the  corresponding  bands  of  neodymium. 

The  two  bands  X4450  and  X4650  of  praseodymium  nitrate  widen  about  20 
A.u.  with  increase  in  concentration  in  very  concentrated  solutions.  In  the 
more  dilute  solutions  there  is  no  change. 

The  X4700  band  of  uranyl  chloride  shows  marked  widening  with  increase 
in  concentration,  especially  towards  the  red  end  of  the  spectrum.  The 
X4900  band  also  shades  off  rapidly  towards  the  red  end  of  the  spectrum. 
When  more  dilute  solutions  were  used,  both  X4600  and  X4700  gradually 
widen  with  increase  in  concentration. 

The  X4700  band  of  uranyl  bromide  widens  uniformly  with  increase  in  the 
concentration.  When  a  more  dilute  solution  was  employed  as  the  starting- 
point,  none  of  the  bands  changed  with  dilution. 

The  absorption  of  concentrated  solutions  of  uranyl  nitrate  is  complete  to 
X4500.  With  increase  in  concentration  this  gradually  recedes  towards  the 
red,  amounting  to  as  much  as  100  a.u.  The  X4700  band  widens  under  the 
conditions  about  20  a.u.  The  sharp  band  X4875  widens  slightly  with 
increase  in  concentration. 

In  the  more  concentrated  solutions  of  uranyl  salts  many  of  the  bands 
change  with  change  in  the  dilution,  while  in  the  more  dilute  solutions  there 
is  scarcely  any  change  at  all. 

The  introduction  of  the  radimnicrometer  into  this  work  converted  it  into  a 
quantitative  study  of  the  absorption  spectra  of  solutions.  The  grating 
spectroscope  and  photographic  method  were  very  efficient  in  locating  the 
positions  of  the  absorption  lines  and  bands  from  wave-lengths  X2000  to 
about  X7600 ;  and  the  photographic  method  gave  some  approximate  idea  as 
to  the  intensities  of  the  various  lines  and  bands.  This  method  is,  however, 
only  roughly  quantitative,  and  is  very  limited  in  the  range  of  wave-lengths 
to  which  it  can  be  applied. 

The  radiomicrometer  provides  us  with  a  quantitative  method  for  studying 
the  intensities  of  the  various  lines  and  bands,  and  also  greatly  extends  the 
range  of  wave-lengths  that  can  be  studied.  In  the  earlier  work  mth  the 
radiomicrometer  much  time  and  labor  were  expended  in  perfecting  the 
instrument,  especially  in  constructing  a  sensitive  radiomicrometer  with  a 
short  period.     Dr.  Guy  accomplished  this  very  successfully. 

The  earlier  work  was  practically  limited  to  the  study  of  the  absorption 
spectra  of  solutions  of  neodymium  salts — ^neodymium  chloride,  bromide, 
nitrate,  and  acetate. 

The  results  were  plotted  in  what  is  known  as  transmission  curves,  which 
express  the  percentage  transmission  for  the  different  wave-lengths.  Solu- 
tions of  different  concentrations  of  a  given  salt  were  studied,  the  depth  of 


GENERAL   SUMMARY   OF  RESULTS.  89 

layer  of  the  solution  varying  inversely  as  its  concentration.  The  product  of 
the  depth  of  layer  times  the  concentration  is  a  constant.  If  the  solvent 
plays  no  r61e  in  the  absorption,  then  the  transmission  curves  for  the  different 
concentrations  of  any  given  salts  must  fall  directly  over  one  another — the 
different  curves  would  be  the  same  curve. 

We  found  that,  in  general,  the  more  concentrated  the  solution  the  less  the 
transparency  and  the  broader  the  absorption  bands;  this  is  exactly  what  we 
obtained  mth  the  grating  spectroscope  and  the  photographic  method.  But 
in  the  more  dilute  solution  the  intensity  of  the  bands  was  greater.  We 
observed  further,  that  with  increase  in  dilution  the  middle  of  the  band  is  dis- 
placed towards  the  longer  wave-lengths. 

The  same  general  changes  with  dilution  in  the  absorption  spectra  of  solu- 
tions of  neodymium  bromide  were  observed  as  with  the  chloride;  the  more 
dilute  the  solution  the  narrower  and  more  intense  the  bands. 

The  bands  of  neodymium  nitrate,  in  general,  show  the  same  changes  with 
dilution  as  those  of  the  chloride  and  bromide.  With  increase  in  dilution  the 
intensities  of  the  bands  increase,  and  their  centers  are  displaced  somewhat 
towards  the  longer  wave-lengths. 

The  three  salts  of  neodymium,  then,  all  show  an  increase  in  intensity  with 
dilution.  A  possible  explanation  of  this  phenomenon,  based  upon  reson- 
ance, has  been  offered.  It  is  a  well-known  fact  that  a  resonator,  when  excited 
by  vibrations  from  a  single  vibrating  resonator  having  the  same  pitch, 
vibrates  more  strongly  than  when  set  into  vibration  by  a  large  number  of 
resonators,  one  of  which  has  the  same  pitch  as  its  own  and  the  others  slightly 
different  periods.  In  a  word,  if  several  vibrators  are  near  together,  every 
one  exerts  a  certain  influence  on  the  others.  The  result  is  that  no  one  of 
them  has  exactly  the  same  period  as  the  original  resonator.  Each  resonator 
damps  the  other  and  we  have  less  perfect  resonance. 

In  a  concentrated  solution  the  resonators  are  relatively  close  together  and 
mutually  affect  one  another.  The  result  is  imperfect  resonance  and  the 
absorption  bands  are  less  intense  in  the  more  concentrated  solution. 

In  the  more  dilute  solution  the  vibrators  are  farther  removed  from  one 
another  and  are  surrounded  by  large  amounts  of  water  of  hydration.  The 
damping  effect  would  thus  be  diminished.  In  such  cases  we  would  have  more 
perfect  resonance  and  the  resulting  absorption  bands  would  be  more  intense. 

Subsequent  work  has,  however,  shown  that  a  part  of  this  effect  can  possibly 
be  explained  as  due  to  the  fact  that  the  slit-width  used  was  not  infinitesimal. 

It  was  found  by  the  radiomicrometer,  as  with  the  grating  spectroscope 
and  photographic  plate,  that  for  a  given  concentration  the  acetate  absorbs 
much  more  than  any  other  salt  of  neodymium. 

One  of  the  most  interesting  facts  thus  far  established  by  means  of  the 
radiomicrometer  is  the  effect  of  the  dissolved  substance  on  the  absorption 
spectra  of  water.  We  noted  that  aqueous  solutions  of  hydrated  salts 
were  often  more  transparent  than  pure  water.  This  is  obviously  a  very 
remarkable  fact,  and  we  at  once  took  up  its  careful  study.    We  compared 


90  ABSORPTION   SPECTRA   OP  SOLUTIONS. 

the  absorption  of  aqueous  solutions  of  strongly  hydrated  salts  with  the 
absorption  of  a  layer  of  water  equal  in  depth  to  the  water  in  the  solution 
through  which  the  light  was  passed.  We  then  carried  out  similar  experi- 
ments with  salts  which,  in  the  presence  of  water,  combine  with  only  a  small 
amount  of  it.  In  a  word,  we  compared  the  absorption  of  light  by  water  with 
the  absorption  of  an  equal  depth  of  water  in  aqueous  solutions  of  strongly 
hydrated  salts,  and  the  absorption  of  light  by  water  with  an  equal  depth  of 
water  in  aqueous  solutions  of  salts  which  are  scarcely  hydrated  at  all. 

The  nonhydrated  salts  with  which  we  worked  were  potassium  chloride, 
ammonium  chloride,  and  ammonium  nitrate.  It  was  necessary  in  all  of  this 
work  to  choose  salts  which  themselves  have  little  or  no  absorption  in  the 
region  in  which  water  absorbs,  i.  e,,  in  the  infra-red.  It  was  found  that 
aqueous  solutions  of  the  above-mentioned  compounds  showed  the  same 
absorption  of  light  as  water  having  the  same  depth  as  the  water  in  the  solu- 
tions in  question.  This  is  exactly  what  would  be  expected.  The  dissolved 
substance  and  the  solvent  do  not  combine  with  one  another  to  any  appre- 
ciable extent,  and  it  would  be  very  difficult  to  see  how  either  could  appre- 
ciably affect  the  absorbing  power  of  the  other. 

When  we  turn  to  the  strongly  hydrating  salts,  very  different  relations 
manifest  themselves.  The  salts  of  this  class  that  were  studied  were  cal- 
cium chloride,  magnesium  chloride,  and  aluminium  sulphate. 

In  the  case  of  a  5.3  normal  solution  of  calcium  chloride,  the  solution  is  the 
more  transparent  from  0.9^t  nearly  to  1/z.  The  water  then  becomes  the  more 
transparent  for  a  short  distance.  From  1.05/z  to  1.2/zthe  solution  is  the 
more  transparent,  becoming  as  much  as  25  per  cent  more  transparent  than 
the  pure  water.  The  water  becomes  more  transparent  than  the  solution 
only  at  and  near  the  bottom  of  the  "water-hands"  at  approximately  Ijjl.  This 
is  what  we  should  expect  if  the  solute  exerts  a  damping  effect  on  the  absorb- 
ing power  of  water.  When  a  smaller  depth  of  the  solution  of  calcium 
chloride  is  used,  the  water  in  the  region  1.25/z  is  more  transparent  than 
the  solution.  From  this  band  on  to  the  longer  wave-lengths  the  solution 
becomes  more  transparent  than  the  water  until  1.42/x  is  reached,  when  both 
solution  and  water  are  practically  opaque. 

The  results  for  magnesium  chloride  are  essentially  the  same  as  those 
obtained  for  calcium  chloride.  The  main  difference  is  that  from  1.0/x  to 
l.ljit,  in  the  case  of  magnesium  chloride,  the  water  is  more  transparent;  while 
for  calcium  chloride  in  this  region  the  solution  is  the  more  transparent. 
The  difference  between  water  and  the  solution  of  magnesium  chloride  in  this 
region  is,  however,  not  great.  For  wave-lengths  longer  than  l.lju,  the  solu- 
tion of  magnesium  chloride,  like  the  solution  of  calcium  chloride,  is  more 
transparent  than  the  water,  the  difference  for  the  two  salts  being  of  the  same 
order  of  magnitude. 

When  a  smaller  depth  of  layer  of  the  solution  was  used,  the  water  was  the 
more  transparent  from  1.22fjL  to  1.34/i.  For  the  longer  wave-lengths  the 
solution  was  the  more  transparent. 


GENERAL  SUMMARY  OF  RESULTS.  91 

The  curve  for  aluminium  sulphate  brings  out  this  new  feature;  at  Ijj,  the 
solution  is  more  transparent  than  the  water.  Beyond  1.04/z  the  water  is 
transparent  to  1.17 jjl,  beyond  which  the  solution  is  the  more  transparent,  as 
with  magnesium  and  calcium  chlorides. 

In  the  region  1.2/i  water  is  the  more  opaque  when  a  shallower  layer  of  solu- 
tion is  used.  From  1.29/jl  to  1.36/z,  water  is  the  more  transparent;  beyond 
1.36jLt  the  solution  is  the  more  transparent. 

The  explanation  of  these  remarkable  results  is  that  they  must  be  due  to 
some  action  of  the  dissolved  substance  on  the  solvent.  That  the  solvent 
can  affect  the  absorption  spectra  of  the  solution  was  first  shown  by  Jones  and 
Anderson  ;i  and  a  large  number  of  examples  of  this  same  action  has  since 
been  found  by  Jones  and  Strong.^  The  action  was  satisfactorily  explained 
as  due  to  a  combination  of  the  solvent  with  the  dissolved  substance,  and  this 
explanation  accounted  for  many  facts  which  could  not  be  otherwise  satis- 
factorily explained.  This  theory  of  solvation  in  solution  has  aided  us  in 
explaining  many  phenomena  which  the  theory  of  electrolytic  dissociation 
alone  could  not  account  for,  as  has  frequently  been  pointed  out. 

The  same  solvate  theory  of  solution  seems  to  aid  us  in  explaining  the  facts 
just  discussed.  Those  substances  that  do  not  form  hydrates  when  in  the 
presence  of  water  show  normal  results  as  far  as  absorption  spectra  are  con- 
cerned. Their  solutions  have  the  same  absorption  as  so  much  pure  water, 
the  substance  itself  showing  no  absorption. 

It  is  the  hydrated  salts,  and  only  these,  which  give  the  abnormal  results 
herein  recorded.  The  combined  water  seems  to  have  less  power  to  absorb  light 
than  free  or  uncombined  water.  This  would  account  for  all  of  the  facts 
observed. 

It  should  be  noted  that  the  presence  of  the  salt  shifts  the  absorption  of  the 
water  towards  the  longer  wave-lengths.  It  was  earlier  observed  that  rise  in 
temperature  and  increase  in  concentration  shifted  the  absorption  of  the  salt 
towards  the  longer  wave-lengths.  The  effect  of  rise  in  temperature  and  of 
increase  in  concentration  is  to  simphfy  the  hydrates  existing  in  the  solution. 
This  simplified  resonator  shifts  the  absorption  towards  the  red.  The  effect 
of  the  salt  on  the  absorption  of  the  water  is  the  same  as  rise  in  temperature 
and  increase  in  the  concentration  of  the  solution  on  the  absorption  of  the 
dissolved  substance.  It  may  be  that  the  dissolved  substance  diminishes  the 
association  of  the  solvent  and  thus  simplifies  the  resonator.  This  may  be 
true  especially  with  the  water  of  hydration  or  the  water  combined  with  the 
dissolved  substance. 

This  new  line  of  spectroscopic  evidence  bearing  on  the  solvate  theory  of 
solution  is  regarded  as  probably  the  most  direct  that  we  have  or  can  hope  to 
obtain  in  favor  of  the  view  that  there  is  combination  between  solvent  and 
solute. 

In  studying  the  absorption  spectra  of  salts,  the  intensity  of  the  light  after 
passing  through  the  solution  of  the  salt  in  question,  was  compared  with  the 

1  Cam.  Inst.  Wash.  Pub.  110.  » Ibid.,  13J  and  160. 


92  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

intensity  of  the  light  after  passing  through  a  depth  of  water  equal  to  the 
water  in  the  solution  in  question. 

The  absorption  spectrum  of  neodymium  chloride  shows  three  pronounced 
minima  representing  the  three  absorption  bands  with  their  centers  near 
X7300,  X7950,  and  X8700,  and  less  pronounced  bands  near  X7150  and  X9000. 
The  latter  may  be  due  in  part  to  the  solvent;  for  all  four  dilutions  studied 
the  X7300  and  X7900  bands  show  complete  absorption  over  a  considerable 
range  of  wave-lengths.  The  minimum  of  band  X8700  is  gradually  lowered 
with  increasing  dilution. 

The  maximum  transmission  of  solutions  of  neodymium  chloride  occur 
near  X7600  and  X8400,  these  solutions  becoming  almost  completely  trans- 
parent beyond  Ijjl,  except  for  the  absorption  of  the  solvent  of  this  region.  It 
seems  that  Beer's  law  holds,  in  general,  for  the  infra-red  absorption  of  solu- 
tions of  neodymium  chloride. 

The  minima  in  the  curves  for  the  more  dilute  solutions  of  neodymium 
chloride  are  in  about  the  same  positions  as  those  in  the  more  concentrated 
solutions,  but  the  solutions  being  more  dilute  are  more  transparent;  hence 
the  minima  are  not  so  pronounced.  The  maximal  absorption  occurs  near 
X7300  and  X7900.  The  X8700  band  has  a  minimum  transmission  of  64  per 
cent.  There  is  complete  transmission  in  the  regions  X7200,  X7600,  X8300, 
and  X9300.  The  drop  in  all  of  the  curves  beyond  0.9jLt  is  due  to  the  absorp- 
tion of  the  water. 

With  further  increase  in  the  dilution  of  the  solution,  there  is  a  lowering  of 
the  maxima.  The  change  is  most  pronounced  in  the  X8700  band,  and  here 
the  absorption  of  the  water  is  the  most  pronounced.  The  absorption  of  the 
water  together  with  the  correction  for  slit-width  may  account  for  this  change, 
and  Beer's  law  may  hold  for  the  dilute  solutions  of  neodymium  chloride 
almost  as  well  as  for  the  more  concentrated. 

Neodymium  nitrate  shows  three  minima  at  X7300,  X7950,  and  X8750. 
The  nitrate  bands  are  not  as  intense  as  those  of  the  chloride,  the  solution  of 
the  nitrate  not  being  as  concentrated  as  that  of  the  chloride.  The  nitrate 
bands,  like  those  of  the  chloride,  become  more  intense  with  increase  in 
dilution.  The  absorption  of  water  becomes  more  and  more  pronounced 
beyond  Ijjl. 

The  minima  for  solutions  of  neodymium  acetate  fall  at  approximately  the 
same  positions  as  with  the  chloride  and  nitrate. 

Solutions  of  neodymium  acetate,  as  indicated  by  the  photographic 
method,  show  greater  absorbing  power  than  those  of  either  the  chloride  or 
nitrate.  The  X7000  band  is  slightly  more  intense  in  the  acetate.  The 
more  intense  bands  X7300  and  X7950  show  the  same  tendency  to  have  their 
minima  lowered  with  increasing  dilution.  There  is  a  rapid  increase  in  the 
absorption  near  X9500,  due  to  the  water  present  in  the  solution. 

Solutions  of  praseodymium  salts  are  transparent  in  the  infra-red  as  far  as 
1.5/x,  except  two  verj^  weak  bands  which  fall  in  the  midst  of  the  intense  water 
bands.     Praseodymium  salts  have  two  groups  of  bands,  one  in  the  green 


GENERAL   SUMMARY   OF  RESTJT.TS.  93 

near  X4600  and  another  near  X5900.  We  have  limited  our  investigations  to 
the  latter  band,  on  account  of  the  small  amount  of  energy  transmitted  at 
X4600.  The  curves  representing  the  absorption  of  different  concentrations 
of  praseodymium  chloride  are  identical  to  within  the  limits  of  possible  experi- 
mental error.  The  results  obtained  with  the  radiomicrometer  are  in  accord 
with  those  found  by  the  grating  and  photographic  plate.  The  minimum  in 
each  case  occurs  near  X5900.  The  total  deviation  from  Beer's  law,  as  shown 
by  solutions  of  praseodymium  chloride,  is  within  the  limits  of  experimental 
error. 

The  curves  for  praseodymium  nitrate  show  that  Beer's  law  holds  here  as 
well  as  for  the  chloride. 

Solutions  of  nickel  chloride  show  an  increasing  absorption  from  X5200 
to  X6300,  where  it  is  complete.  Complete  absorption  extends  to  X7200. 
Transmission  increases  to  X9000,  then  decreases  to  zero  at  XIOOOO.  The 
visible  spectrum  of  salts  of  nickel  consists  of  intense  broad  absorption  bands 
in  the  blue  and  red,  having  a  single  region  of  transmission  in  the  red,  extend- 
ing to  about  X6500.  By  means  of  the  radiomicrometer  we  could  study  the 
region  of  transmission  near  X9000.  Beyond  this  we  could  not  go  because  of 
the  absorption  of  the  water. 

The  absorption  of  nickel  nitrate  closely  resembles  that  of  the  chloride. 
There  is  maximal  absorption  at  X5400  and  X9000.  There  is  complete  absorp- 
tion in  the  region  X7000  and  beyond  l.lju. 

The  solution  of  nickel  sulphate  studied  is  sHghtly  more  dilute  than  the 
chloride  and  nitrate.  In  no  region  is  there  complete  absorption.  There  is 
maximal  transmission  near  X5400  and  X9000,  and  minima  at  X6900  and 
XI 1000.  Readings  were  not  extended  beyond  this  region  on  account  of  the 
intense  absorption  of  the  water.  The  three  salts  of  nickel  studied  have  just 
about  the  same  absorption  spectra,  the  curves  sho^ving  maxima  and  minima 
in  just  about  the  same  regions  of  the  spectrum. 

Salts  of  cobalt  in  the  visible  region  have  a  strong  ultra-violet  absorption. 
There  is  a  band  in  the  orange  near  X5000,  and  increasing  transmission 
towards  the  red.  The  infra-red  absorption  of  solutions  of  cobalt  salts  was 
studied,  and  the  absorption  of  the  chloride,  bromide,  nitrate,  sulphate,  and 
acetate  compared.  The  transmission  curves  for  all  of  these  salts  have 
maxima  at  X5950,  X7800,  X9ld0,  and  XIOGOO. 

The  transmission  curves  for  all  of  the  salts  of  cobalt  studied  rise  rapidly 
from  X5000  to  X5900.  The  curves  show  a  broad,  slight  absorption  over  the 
region  near  X6500,  and  reach  a  maximum  transmission  from  X7000  to  X8000. 
There  is  a  series  of  small  absorption  regions  near  X8400,  X8900,  and  X9800. 
Beyond  X10500  the  absorption  increases  rapidly  to  the  region  where  water 
is  practically  opaque. 

The  curves  for  cobalt  sulphate  are  slightly  better  defined  than  those  for 
the  other  cobalt  salts. 


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Book  Slip-50m-8,'66(G5530s4)458 


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Jones,  H.C. 

The  absorption  spectra 
of  solutions. 

PHYSICAL 
SCIENCES 
LIBRARY 


QC437 

PSL  Aiint 


j 
i 

LIBRARY 

UNIVERSITY  OF  CALIFORNIA 

DAVIS 

if74287 

Call  Number: 

QC437 

Jones,   H.C. 

The  absorption  spectra 
of  solutions. 

J78 

